If your bike has one gear on the back and one on the front, you may be able to adjust the chain tension by moving the rear wheel back in the dropouts-the part of the frame that the wheels are touching. Otherwise you may need a new chain.
If you have a bike with multiple gears, you may have to adjust the low adjustment on the front deraileur - the thing that moves the chain on and off the front set of gears.To make the adjustment look for a pair of phillips head screws.
Provided that you have not made any adjustments, do this:
The low adjustment is usually the one that is closest to the frame or the lower of the two. Turn it clockwise a quarter turn. Get on the bike and test it. If that does not work, turn it back because you probably need to adjust the cables, especially if they have not been lubed. This however, is a little more complicated and you will need to say what kind of shifters you have.
