A couple of quick troubleshooting tricks. Take a set of jumper cables and hook the red to the + on the battery and the other red end to the power lug on the starter. It should crank when you touch it. If it doesn't the starter is bad, unless the ground on the starter is bad. To test that you hook the black to the negative on the battery and the other black to the case of the starter.
If it cranks that way, there are two common problems. The starter relay can cause the problem. It is where the large cable from the starter connects. Another large lead from the battery also connect to it. Just to check out the wires aren't corroded or broken, jump the two cable terminals on the relay with a jumper cable or I've used the handles on a pair of pliers. If it doesn't crank then, but it did with the jumpers, your battery cables are corroded or broken.
If I remember right, there are two green push on terminals on the starter relay. I pull the cable off of the lower connector and jumper the terminal to the battery plus. If the starter and cables are fine, as well as the relay, it should crank.
If that last step passes, you have an electrical problem on the control side, not the power side. Things in that loop are the keyswitch, wiring, bulkhead connector, and on an automatic, the Neutral Safety Switch.
Good luck.