It could be so many things it's hard to say without actually being able to do some basic tests. Make sure the battery and alternator cables are all clean tight and secure. If you have a volt meter put a voltmeter across the battery at rest if it is fully charged it should be 12.6 volts if not the battery needs to be charged and tested. With the engine running the voltage across the terminals of the battery should read approx. 14.6 volts if the alternator is charging correctly. Does the battery light come on when you turn on the ignition key, go out when the engine is running and remain out when the headlights are turned on?