If you bought the bike from a local bike shop, bring it back to them to address the problem. Most if not all bike shops provide at least one free tuneup within 90 days of purchase -- and will do quick adjustments when needed for individual problems that arise within that period.

If you bought the bike used, you might be dismayed at some of the responses you're going to get here. There are a number of possible causes of that problem, many of which can't be confirmed without going to a bike shop and paying a mechanic to fix the bike.