First step would be to check for clean battery terminals and connections at both the block (ground) and starter. There may be enough current for lights, but not for the starter. If that shows good, jump the connector on the starter: On the solenoid, you will see one small wire connected via a female spade terminal (square insulator) which slides over a male spade on the back of the solenoid. Jump 12 volts to that terminal. If the engine cranks, your problem is not the cables or the battery...or the starter. Next, do this with the ignition on. If the car starts, the ignition circuits in the starter key are good. Your next step, in my opinion, would be to remove the electrical connector block on the back of your ignition switch and examine for bad contacts in the "crank" circuit. That is, the circuit that send current to that terminal on the solenoid. The ignitions switch contact block is prone to problems. Good chance you will find your trouble there.

DS