Chris, What we need to start with is checking the voltage at the starter. You should have 12 volts at the starter.
Check all the battery connections for corrosion and that they are very tight.
If yes then with a jumper wire jump from the yellow/red terminal # XXXXX on the starter to battery positive # XXXXX
Does the engine crank? If yes we know the engine is ok. If no we need to check the battery cables and connections and test the voltage at the battery terminal # XXXXX while cranking must be 9.5+ volts. If voltage then test for voltage to the starter # XXXXX when cranking. If voltage starter is bad if no voltage the solenoid is bad.
Now we need to check for voltage at the key switch red terminal # XXXXX If no voltage check the fuse next to the start solenoid. If voltage then we need to check for voltage at the yellow/red terminal # XXXXX when cranking.
If voltage then we need to go to the start solenoid and check for voltage at the yellow/red term # XXXXX when cranking. If voltage and the engine doesn't crank the solenoid is bad. If no voltage then the neutral safety switch is bad.
Here is a diagram to follow. If you have any questions on the procedure just post back and I will see if I can be clearer.
Post back with questions, findings and concerns before rating please.