Put a volt meter across the batter, a good battery puts out 12.8 volts. If it less Check the electrolyte level in the battery, it should be just below the holes. Charge the battery for a while, and check voltage again. If voltage doesn't get above 12, get a new battery. Clean the battery posts and cables. It may look clean, but that doesn't mean it conducts electricity. If none of this works, I would start looking at the solenoid and starter.