1 check battery with multi meter with car not running. should have at least 12 to 12 and a half volts.
2 check battery with car running should be at least 12 to 14 volts.
3 car running pull off one of the battery cable from either positive or negative while car is running.
4 if car dies bad alternator
5 like everybody has said you must start with a good battery supply source.
6 also make sure that there is a tight connection on both positive and negative posts; and that all connections are clean and free of corresion. this sounds simple but it is the most important first step. you must start at a and then b and c if you skip you will cause yourself a lot of grief. if you have a good scanner of your own then try pulling codes. if not then go to auto store and see if they will pull and give them to you. or dealer will for fee pull them. once you have done all or a least made sure you have good battery get back to me and i will give you some other avenues to try that are fairly easy to do and not overly time consuming.