It's easy to fall into the trap of thinking that getting rich in the stock market is only possible for those who are already wealthy. While it is easier to generate wealth when you have a lot of money to invest, you don't need to be rich to become a stock market millionaire.

In fact, you don't even need to be an experienced investor. Choosing the right investments is critical to maximizing your earning potential, but it's easier than you may think to retire rich. By investing just a few hundred dollars per month in this particular ETF, you could become a multimillionaire someday.

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Where to invest your money

You have countless options when it comes to choosing where to invest. One of those options is exchange-traded funds, or ETFs.

An ETF is a group of stocks consolidated into a single investment. So when you invest in just one ETF, you're instantly investing in hundreds or even thousands of different stocks. This will help diversify your portfolio and reduce your risk.

While there are many different ETFs to choose from, a solid option is the Vanguard S&P 500 ETF (NYSEMKT:VOO). This fund tracks the S&P 500 index, meaning it includes the same stocks as the index itself and mirrors its performance.

The S&P 500 is one of the best representations of the stock market as a whole, which can further limit your risk. The stock market is prone to volatility, but it has a long track record of bouncing back after crashes and corrections. Because this ETF follows the market, it's very likely to recover from downturns as well.

Also, by investing in the S&P 500, you're buying some of the largest and most successful companies in the U.S. A few of the biggest names in the S&P 500 include Apple, Microsoft, Alphabet, Amazon, and Facebook.

Becoming a multimillionaire

Regardless of where you invest, your exact returns will vary depending on how the market performs. However, since its inception, the S&P 500 has earned an average rate of return of around 10% per year.

This doesn't necessarily mean you'll earn a 10% return every single year with this investment. Over the past year, for example, the Vanguard S&P 500 ETF has earned a return of more than 40%. There will be other years when you earn much lower returns or even experience losses. Over time, though, those yearly returns should average out to roughly 10% per year.

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Say you're just getting started investing, and you invest $400 per month while earning a 10% average annual return. After 40 years, you'd have around $2.124 million.

Of course, 40 years is a long time to invest. If you don't have that much time to save, you'll need to increase the amount you're investing each month. For instance, if you only have 35 years to save, you'd need to invest around $650 per month to accumulate at least $2 million.

On the other hand, if you're able to leave your money invested for more than 40 years, you won't need to save quite so much each month to become a multimillionaire. By investing for 45 years, for example, you'll need to save just over $225 per month to reach $2 million in total savings.

While getting rich in the stock market takes time, investing in the Vanguard S&P 500 ETF could help you reach your goal. By simply investing consistently and giving your money as much time as possible to grow, you can earn more than you may think.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.