When I try to start it

If the starter motor won't crank, have to check the starter motor wiring circuit. Battery has a full charge and battery connections all clean and tight? If the starter motor won't crank, some possible issues, ignition switch--neutral or clutch switch depending on your tranny--the starter itself. Another possible issue is your security system, where applicable. Your engine isn't froze-up, is it?

Any testing at the starter, make sure tranny is in park or neutral and the parking brake is set.

As you can see from diagram, your security system can be a problem.