Do you have a multimeter to check voltage? if you do check for 12 volts at the heavy cable at the starter/solenoid. If good have someone try starting the car while checking the voltage, the voltage will drop but it should be no less than 9-10 volts. If less inspect the battery to starter cable. If you have a clamp on battery terminal (the type that has two bolts to clamp the terminal to the cable) that is probably where the problem is. Make sure you have a good clean connection or replace the clamp or cable. Also make sure both battery posts are clean and terminals tight. Check that the battery ground cable is tight and clean at the engine/frame.
Clifton