First check to make sure that you have power to your starter solenoid. You can check this by going between the big wire coming from the battery and ground(engine block). If you don't have voltage, your engine is not properly connected to the battery bank (could be ground or hot but it should be a direct wire run for each).
If that is good, check the other side of the solenoid when you are trying to crank. You should see something like 8V or so between the switched side of the solenoid(the big wire/bar going to the actual starter) and ground when cranking. Chances are, if your engine won't crank the 2nd test won't work and the first one may or not.
If the 1st test was good and the 2nd was not, you need to check whether the solenoid is being energized. Check voltage across the two small wires on the solenoid when trying to crank. If there is no power there, then your problem is either in your key switch or low current wiring. If the solenoid is being energized but you failed test 2, you probably need a new solenoid.
I hope that this provides a good place to start. The key is to systematically rule out potential problems using a multimeter.