O'Leary advises you funnel whatever money you can into investments that will grow over time.
"This is advice I'll give you no matter what age you are," O'Leary says. "When you make money, I don't care if it's a gift for your birthday present, or you have a job on the side, or you're scooping ice cream or whatever it is — you have to take 10 percent of your paycheck every two weeks and invest it.
"People say, 'I can't afford that! I can barely afford my rent!'" he continues. "But it's not true, you buy crap you don't need every day."
Investors, like Mark Cuban and Warren Buffett, recommend investing in low-cost S&P 500 index funds, which is an affordable way to invest in the 500 largest companies in the stock market.
But another way to get started investing if you don't have a lot of money is with exchange-traded funds, known as ETFs, which O'Leary recommends. An ETF is a basket of different assets, and you can find ETFs that include all sorts of asset classes — stocks, bonds, commodities and real estate too. So they have the benefit of diversification, according to O'Leary.
"You should never have all of your eggs in one basket, you should never own just one stock," he says. "There are 11 sectors in the economy, things like energy, defense, technology and there are thousands of stocks."
Still, you'll need to do your homework on the assets that make up the ETF to be sure they are sound investments, he says.
"One of the problems that young investors should consider and be mindful of is that an ETF is only as good as what's inside the basket," O'Leary tells CNBC.
O'Leary founded a company that offers ETFs, called O'Shares ETF Investments, which specifically targets dividend-paying stocks. A dividend is money that a company pays its shareholders, typically every quarter. By investing in stocks that pay dividends, you can spend or reinvest the amount paid out by the company, without having to touch your initial investment, leaving it to grow.
It's a lesson he learned from his mother.
"My mom understood a very basic premise," he explains. "Never spend the principal, only the interest."
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