you'll want to check the voltage at the starter solenoid 'start' terminal when your trying to crank the engine. It must be battery volts. Anything less and you have a resistance problem somewhere that needs to be resolved.
If you are getting battery volts at the solenoid then the problem is most likely the starter solenoid contacts inside the solenoid. The best solution is to remove the starter and take it to a rebuild shop and have them repair it. Or just order a reman starter from your dealer (they will want the old one for a core)