several inexpensive ways to fix this
There are several inexpensive ways to fix this, providing that your door is the correct size for the opening. First is the adhesive foam-type weather stripping. It comes in rolls and can be purchased at Lowes, HomeDepot, and other hardware stores. Also, there are door sweeps (believe that's what they're called) that are a rubber-type material and can be fastened to the bottom of the door, and there's a type that's made to be fastened at the side of the door as well. I'm sure there are others as well, so best to make a trip to the hardware store, ask a clerk and look at your options.