Check the easy stuff first, like battery voltage.
Check that the starter relay in the underhood PDC is clicking when the key is turned to Start. If not, swap with another matched relay and re-test. If still not clicking, could be a bad NSS or problem with the ignition switch.
If the starter relay is clicking but the starter is not turning, try tapping the starter with a piece of wood to loosen any sticky brushes. While you're down there, make sure all the starter electrical connections are solid and check the harness for continuity. If it still doesn't work, try directly jumping the solenoid with a screwdriver. If that doesn't make anything happen, I'd say the starter is probably worn out.