Investing is one of the most effective ways to build wealth. Some experts even say that the only way to "truly grow" your money is to put it to work.

If you have an extra $500 to $1,000 and are ready to dip your toe into the markets, "you should put your money in just one thing: the S&P 500," certified financial planner Peter Mallouk tells CNBC Make It.

The S&P 500 index fund holds 500 of the largest companies in the U.S., from Google to Disney to ExxonMobil. It allows you to take advantage of the success of major corporations without the risks associated with buying individual stocks.

"It's the lowest cost investment that exists," says Mallouk, who is the president of wealth management firm Creative Planning. "You can buy it at a discount custodian, like a TD Ameritrade, Fidelity or Charles Schwab, where the trading fees are exceptionally low. And with those 500 stocks, you now own about 80% of the market capitalization of the entire United States." In other words, an S&P 500 fund covers about 80% of the U.S. stock market, so it's an easy and affordable way for investors to capture core U.S. stock market performance.

Plus, "you have global exposure," Mallouk says, "because these companies get a lot of their earnings overseas: McDonald's has shops in China, and Walmart has locations in Europe. So you wind up with a low cost, diversified portfolio that's invested in the global economy."