You need a rear derailleur adjustment. Assuming that it was never right it could be either a limit screw adjustment or a cable tension adjustment.

Since your bike is so new, I'd advise taking it back to whoever sold it to you. If you want to tackle fixing it yourself, go to the Park tool website and find the instructions for installing a rear derailleur. If you decide to do that, don't mess around. Completely disconnect the shift cable and start from the very beginning. Follow the instructions IN ORDER and don't skip any steps.