There is a voltage regulator somewhere in the engine compartment. Normally these are attached to the alternator but not always. The voltage regulator is easy to find if you trace all the wire from the alternator. A voltage regulator is cheap most of the times well under $20 bucks. You'll find it just mounted to the frame inside the engine compartment. First things first.
1. Make sure the cables on the battery are tight.
2. Make sure the connections on the alternator are tight.
3. Next start the car use a voltmeter put the setting on 20VDC
Place the red lead on the positive termial, place the other lead on the frame of the car (this will give you ground), Check the meter you want to have a reading of 13.5 or above preferably 14.6
4. If the reading is 12 or below then change your voltage regulator.