How to Start Investing With $100: A Plain-English Guide to Index Funds

Picture Dana, a 29-year-old dental hygienist in Columbus, Ohio. She has $100 left after rent, groceries and the car payment, and she keeps hearing that she should be “investing.” Every time she opens a brokerage app, she sees ticker symbols, red and green arrows and a hundred things to click. She closes the app and moves the $100 back to checking. Sound familiar?

Quick version: watch the Web Story for the highlights in under a minute.

Dana is a composite, a made-up but typical example, but the feeling is real. A lot of Americans stay out of the market because it looks like a game for people with money, math degrees or both. It is not. The simplest way to invest is also the one that most professionals quietly recommend for ordinary savers: buy a low-cost index fund and keep buying it.

This guide walks through exactly how to do that with as little as $100. You will learn what an index fund is, why fees matter more than most people think, which account to use first, and the five steps to place your first purchase today. No jargon, no hot stock tips.

What an index fund actually is

An index is just a list. The S&P 500 is a list of 500 of the largest U.S. companies. The Total Stock Market index is a longer list that includes almost every publicly traded U.S. company, from Apple to small regional banks. An index fund is a fund that buys everything on a list, in the same proportions, and holds it.

Nobody is picking winners. There is no manager reading earnings reports and guessing which company will jump next quarter. The fund simply mirrors the list. When one company on the list is added or dropped, the fund adjusts automatically. That “hands-off” design is exactly why the fees are so low.

When you buy one share of an S&P 500 index fund, you own a tiny slice of 500 businesses at once. If one of them has a terrible year, the other 499 soften the blow. That built-in spread is called diversification, and it is the closest thing to a free lunch in investing.

Why index funds beat most people (and most pros)

Here is the uncomfortable truth about stock picking. Every year, S&P Dow Jones Indices publishes a report called SPIVA that compares professionally managed funds against their index. Year after year, in recent editions, the large majority of actively managed U.S. large-cap funds trail the S&P 500 over 10 and 15 year stretches. Over a 15-year window, the share that fall behind has been around 85 to 90 percent in many scorecards. Professionals with research teams, Bloomberg terminals and decades of experience often cannot beat a simple list.

There are two reasons. First, fees. An actively managed fund has to pay its analysts and traders, and those costs come out of your return every single year. Second, markets are hard to predict. Even a manager who is right more often than wrong has to be right enough to cover their own fee.

This does not mean you can never win by picking a stock. It means the odds are stacked against you, and index funds let you skip the contest and still collect the market’s return.

The fee that quietly eats your money

Every fund charges an annual fee called an expense ratio. It is shown as a percentage of what you have invested. A 0.03% expense ratio means you pay $3 a year for every $10,000 invested. A 1% ratio means you pay $100 for the same $10,000. That sounds small until it compounds.

Here is an illustration. Suppose you invest $10,000 once and the market returns 7 percent a year before fees. Nothing else is added.

That is a difference of around $18,000 on a single $10,000 deposit. The numbers are an illustration, not a forecast, since real returns bounce around every year. But the lesson holds: when two funds hold similar things, the cheaper one wins.

Index fund vs. ETF vs. mutual fund: what is the difference?

You will see three terms, and beginners often mix them up. Here is the short version.

• Index mutual fund: You buy it directly from the fund company or your brokerage at the end-of-day price. Some have minimums, though many now have none. Examples include Fidelity’s ZERO and 500 index funds and Vanguard’s Total Stock Market Index Fund.
• Index ETF (exchange-traded fund): The same idea, but it trades like a stock throughout the day. Most brokerages let you buy a fractional share, so you can put in exactly $100.
• Target-date fund: A “set it and forget it” fund that holds a mix of stocks and bonds and slowly gets more conservative as your chosen retirement year approaches. Many 401(k) plans use these by default.

For a beginner, any of these is fine. What matters is that the underlying fund tracks a broad index and charges a very low fee. Many broad index funds from the big providers charge somewhere between 0.00% and 0.10%. Check the current expense ratio on the fund’s page before you buy, since fees can change.

Which account should you use first?

The account you pick matters almost as much as the fund. Think of it as the container that decides how much of your gains the IRS gets to take. Work through the list in this order.

1. Your employer’s 401(k), up to the match. If your employer matches contributions, that is an instant return that nothing else can beat. If they match 50 cents on the dollar up to 6 percent of your pay, put in at least 6 percent. Not doing so is leaving part of your paycheck on the table.
2. A Roth IRA. You contribute money you have already paid tax on, and the growth and qualified withdrawals in retirement are tax-free. For 2026 the IRS contribution limit is $7,500 for most savers, with income limits for high earners. If you want a full comparison, see our guide to Roth IRA vs. Traditional IRA.
3. The rest of your 401(k). If you still have room after the IRA, go back and add more to the 401(k). The 2026 employee limit is $24,500.
4. A regular taxable brokerage account. No limits and no penalty for withdrawing, but you pay tax on dividends and on gains when you sell. This is where flexible money goes once the tax-advantaged accounts are full.

One more thing before investing at all. Make sure you have a small cash cushion. If you do not yet, our walkthrough on building an emergency fund from scratch is the right first step. Money you might need within a few years does not belong in the stock market.

How to invest your first $100: five steps

1. Open the account. Fidelity, Charles Schwab, Vanguard and similar brokerages let you open an IRA or taxable account online in about 15 minutes. You will need your Social Security number, a bank account and your employment details. Always type the web address yourself rather than clicking a link in an email or text.
2. Fund it. Link your checking account and transfer $100. The first transfer may take one to three business days to settle.
3. Pick one broad fund. A total U.S. stock market or S&P 500 index fund is a classic starting point. Search by fund name or ticker, and confirm that the expense ratio is low and that the fund says “index” in its description.
4. Place the order. If you are buying an ETF, choose “dollars” instead of “shares” so the brokerage buys a fractional share. For an index mutual fund, simply enter $100.
5. Automate. Set up a recurring monthly investment, even if it is just $25 or $50. Automation removes the temptation to wait for the “perfect” moment.

That is the entire process. If you feel a little underwhelmed, good. Boring is the point.

A realistic example: Marcus and the $100 a month plan

Let’s meet Marcus, another composite. He is 25, earns about $48,000 as a logistics coordinator in Phoenix, and decides to invest $100 a month in a broad index fund inside a Roth IRA. He does not touch it, does not check it daily and does not add extra.

If the market grows an average of 7 percent a year (a rough long-run assumption, not a promise), Marcus’s $100 a month grows to roughly $122,000 by age 55. He deposits only $36,000 of his own money over 30 years. The rest, about $86,000, is growth. Because it sits in a Roth IRA, he owes no federal tax on that growth when he withdraws it in retirement, assuming he follows the rules.

Now say his pay goes up and he bumps it to $200 a month at age 30. The final number gets a lot bigger. The point is not the exact figure. The point is that starting small and increasing it over time is far more powerful than waiting until you can invest a large amount.

What about market crashes?

This is where most beginners lose money, and it has little to do with which fund they pick. It has to do with what they do when prices fall.

In early 2020 the S&P 500 dropped roughly a third in about a month. In 2022 it entered a bear market again. Each time, anyone who sold in a panic locked in the loss. Investors who kept buying through the drop (and did nothing else) saw prices recover. After the 2020 crash, the index made new highs within the same year.

Past recoveries are no guarantee of future ones, and some downturns, like 2000 to 2002 and 2007 to 2009, took years to heal. That is why the rule is simple: only invest money you will not need for at least five years, and treat a crash as a sale rather than a disaster. A falling market means your monthly $100 buys more shares.

Mistakes that cost beginners the most

• Chasing last year’s winner. A fund or sector that jumped 40 percent often does not repeat. A broad fund spreads the risk.
• Checking your balance every day. It makes every dip feel like an emergency. Check once a quarter.
• Paying hidden fees. Some advisors and brokers push funds with “loads” or 1 percent annual charges. Ask directly: “What is the total annual cost, in percent?”
• Investing money you need soon. Rent, car repairs and a down payment due next year belong in a savings account.
• Forgetting to invest the bonus. When you get a raise, send half of it to your fund before lifestyle spending soaks it up.
• Falling for “guaranteed return” promises. No legitimate investment guarantees profits. Scammers love beginners, and if someone you met online or by text offers one, walk away. See our guide to spotting scams for the warning signs.

Do you owe taxes on index funds?

It depends on the account. In a Roth IRA or a 401(k), you do not owe tax each year on dividends or trades. In a regular brokerage account, you will receive a tax form (a 1099) each winter showing dividends and any gains you realized. If you hold a fund for more than a year before selling, the profit is generally taxed at the lower long-term capital gains rate. Index funds also tend to be tax-friendly because they trade rarely. Tax rules change, so check the IRS website or ask a tax professional for your situation.

Frequently asked questions

Can I really start with $100?

Yes. Many brokerages have no account minimum, and fractional shares let you buy a slice of an ETF for any dollar amount. Some index mutual funds have no minimum either.

Is an S&P 500 fund or a total market fund better?

Both are solid. The S&P 500 holds about 500 large companies, while a total market fund adds mid-size and small ones. Their long-term results are very similar. Pick one and stay with it.

Should I buy international funds too?

Some investors add an international index fund for diversification. If you want to keep things simple, a single U.S. fund or a target-date fund is a reasonable beginning.

What if the market drops right after I invest?

Then your next deposit buys more shares at a lower price. If you will not need the money for years, a short-term drop matters much less than it feels like it does.

Do I need a financial advisor?

Not to buy a basic index fund. If your situation is complicated, such as owning a business, receiving an inheritance or approaching retirement, a fee-only fiduciary advisor can be worth it. Ask how they are paid before you sign anything.

The bottom line

Dana, the hygienist from the start of this story, opened a Roth IRA that weekend. She put in her $100, picked a broad index fund and set up an automatic $75 deposit for the first of each month. It took less than 20 minutes. Her balance did not change her life that month. But she stopped feeling like investing was something that belonged to other people.

That is the real value of starting small. The habit matters more than the amount, and time does the heavy lifting. Open the account, pick a low-cost fund, automate it and then leave it alone.

This article is for general education and is not personalized financial advice. Investing involves risk, including loss of principal. Example figures are illustrations based on assumed returns, not predictions. Check current fees, limits and rules with your brokerage or a qualified professional.