The problem is not so simple as there maybe more that one thing going wrong. 1st and most simple is to have the battery checked. 2nd would be to have your alternator checked, this is the most likely problem. If it is the alternator you are looking at a few 100 dollars for the fix.
There are a few things that may cause this to happen, the most common are a bad battery, altinator, and caroded battery posts. First try cleaning the posts on top of the battery with a wire brush and charging the battery overnight. If that does not work replace the battery since this is cheaper then buying a new altinator.
In most cases the battery is defective and needs to be replaced. The vehicle should be checked to verify that there are no electrical shorts and the charging system is functioning correctly.
The first thing to check when a car wont hold a charge is the battery. you can remove your battery easily and take it to a battery store or some hardware stores and have them put a load on it and tell you if it is bad.