2018 was the best start to a New Year for stocks since 2006, leaving many wondering if it's too late to buy U.S. stocks.

The solution? Invest in the markets gradually. That's the assessment from JJ Kinahan, chief market strategist at TD Ameritrade.

"If you want to buy, set a price level and buy something there," he said, adding that investors should remain cognizant of the average purchasing price of their securities. 

More From TheStreet: