If you've ever asked the internet how to start investing, you've probably gotten the same three words back: "buy index funds." It's the advice from finance experts, your coworker, and probably a billionaire or two. But almost nobody explains what an index fund actually is — or why low-cost index funds are often treated as a strong starting point for long-term investors.
Here's the plain-English version: what an index fund is, why costs matter so much, and how a beginner can buy their first one.
What Is an Index Fund?
An index fund is a single investment that holds a basket of securities designed to match a market index — a rules-based list or benchmark built to represent a segment of the market.
An index fund can track many different indexes: the S&P 500, the total U.S. stock market, international stocks, bonds, or a target-date-style mix. Some index funds hold stocks, some hold bonds, and some hold a blend. The S&P 500 is just the most famous example.
Take that S&P 500 fund. The S&P 500 tracks roughly 500 leading large U.S. companies, and its exact holdings and weights change over time. You can't buy "the S&P 500" directly. Instead, you buy an S&P 500 index fund, and that fund holds those companies (a bit more than 500 in stock terms, since some have multiple share classes) so it can follow the index for you. Put in $100, and you get instant exposure to hundreds of companies through the fund.
One important nuance: most S&P 500 index funds are market-cap weighted, meaning larger companies make up a bigger share of the fund. So "500 companies" doesn't mean you own an equal slice of each — diversified does not mean evenly spread. In a market-cap-weighted fund, the largest companies can drive a large share of both the returns and the risk.
Why Index Funds Are So Popular
Index funds don't try to beat the market. They try to match it — and that's popular for a few reasons: low cost, built-in diversification, simplicity, and the sheer difficulty of beating the market consistently.
That last point surprises people. Broad, low-cost index funds have historically outperformed many actively managed funds after fees. S&P's SPIVA U.S. Year-End 2025 scorecard reported that 79% of active large-cap U.S. equity funds underperformed the S&P 500 in 2025, and underperformance has been even more common over longer periods in many SPIVA measurements. (SPIVA also accounts for funds that disappear or merge — which matters, because poor performers don't always survive long enough to show up in casual comparisons.)
None of that means active management is useless. Some active managers do outperform, and active strategies can be used for reasons beyond beating the S&P 500. The hard part is identifying the future winners in advance, after fees and taxes.
The One Number That Matters Most: The Expense Ratio
When you compare index funds, one of the most important numbers is the expense ratio — the annual fee, shown as a percentage.
The math is simple. A 0.03% expense ratio means you pay $3 per year for every $10,000 invested. Many actively managed funds charge 0.5% to 1% or more — at 1%, that's $100 per year on the same $10,000, and that gap compounds every year.
The good news: major S&P 500 index funds are very low-cost. As of 2026, a few common examples:
- VOO (Vanguard S&P 500 ETF): 0.03%
- FXAIX (Fidelity 500 Index Fund): 0.015%
- IVV (iShares Core S&P 500 ETF): 0.03%
(Verify current expense ratios before buying — fund fees can change.)
When funds track the same index closely and are equally accessible in your account, a lower expense ratio is often attractive. But it isn't the only factor. Also weigh tracking error (how closely the fund follows its index after fees), trading costs, minimums, tax efficiency, and whether the fund is easy to automate. In a taxable brokerage account, tax efficiency and capital-gains distributions can matter — ETFs are often tax-efficient, but account type and fund structure matter too.
Index Fund vs. ETF vs. Mutual Fund
These terms get tangled, so here's the short version:
- An index fund is any fund that tracks an index. It's a strategy, not a wrapper.
- That strategy comes in two main wrappers: a mutual fund or an ETF.
A few practical differences:
- ETFs trade intraday like a stock; mutual funds trade once per day after the market closes.
- ETFs are often buyable in fractional shares at many (not all) brokerages; some brokerages make automating easier with mutual funds.
- With ETFs, be aware of bid-ask spreads and the small chance an ETF trades slightly above or below its underlying value (usually tiny for large, liquid ETFs).
- Mutual funds may carry minimum investments or transaction fees at some brokerages.
- In a 401(k), the index option is often a mutual fund or collective investment trust rather than an ETF ticker you'd buy in a brokerage account.
For many beginners using a taxable brokerage account or IRA, a broad low-cost ETF can be a simple starting point. In a 401(k), you usually choose from the plan's menu.
And remember: an S&P 500 fund covers large U.S. companies, not the entire global market. A total U.S. stock market fund, an international index fund, a bond index fund, or a target-date fund may fit better depending on your goal, timeline, account type, and risk tolerance.
How to Buy Your First Index Fund
First, make sure you're ready to invest. Near-term cash needs and high-interest debt should be handled well enough that you can leave this money invested for years.
Then:
- Choose the right account. A taxable brokerage account for flexible investing, a Roth or traditional IRA for retirement (if eligible), or your employer plan for workplace savings. Contribution limits, income limits, withdrawal rules, plan menus, and tax treatment all vary — a Roth IRA or 401(k) is often a smart home for long-term index investing.
- Pick a fund. Compare expense ratios (and the factors above) and choose one that fits your goal and account.
- Actually buy it. After transferring cash, place the buy order or set the investment election — cash sitting in the account is not the same as being invested. For large, liquid ETFs during market hours, a market order is usually fine; use a limit order if you want more price control, and avoid trading when markets are closed or unusually volatile.
- Automate what you can. If your brokerage supports recurring investments, set one for an amount your budget can handle through ups and downs. Consistent buying is what lets compounding do its work. Turn on dividend reinvestment if it fits your plan, and if you hold more than one fund, rebalance occasionally so your mix doesn't drift.
Thanks to fractional shares at many (not all) brokerages, you can often start with just a few dollars. You don't need to wait until you have "enough."
A Few Beginner Mistakes to Avoid
- Assuming the S&P 500 equals full diversification. It's U.S. large-cap exposure — not mid- and small-caps, international, or bonds.
- Buying overlapping funds without realizing they hold mostly the same stocks.
- Leaving cash uninvested after depositing it.
- Ignoring account type and taxes — the right fund can depend on whether it's in an IRA, 401(k), or taxable account.
- Investing money you'll need soon. Index funds are for the long term.
- Confusing price per share with "cheapness." A $50 share isn't cheaper than a $500 share; the expense ratio and what the fund holds are what matter.
- Buying leveraged or niche products as if they were plain index funds.
- Chasing last year's winner or trying to time the market. For long-term money, staying invested has historically been more reliable than repeatedly trying to jump in and out at perfect moments — and selling during a downturn can lock in losses and interrupt the plan.
The Bottom Line
A broad, low-cost index fund can be one of the simplest ways to start long-term investing. For some investors, that's an S&P 500 fund; for others, a total-market, international, bond, or target-date option may fit better. The common thread is low cost, broad exposure, and the discipline to keep contributing and leave it alone.
You don't need to pick winners or have a finance degree. You need the right account, a low-cost fund that fits your goal, and a contribution you can stick to.
That's what clarity looks like.
Canopy can help you view your supported connected and manually entered investment accounts, cash, debts, goals, and an estimated net worth in one place, so investing decisions are easier to weigh alongside the rest of your financial picture. Canopy doesn't recommend specific funds, place trades, provide investment advice, or determine the right allocation for you. Start with Canopy — free, no credit card needed.
Related Reading
- How to Start Investing With $100 in 2026
- How Compound Interest Works: Why Starting Early Matters
- What Is a Roth IRA? The 2026 Rules and How to Open One