Hook a dvm to the battery and take a reading when you engage the starter, I bet you have a bad battery, it can read 12 whatever volts at rest ( nothing on ) and when you turn the key to start the battery volts drop. Like a rock!
This is all covered in the 'no crank' link I posted.
This also is dependent upon good connections and wires. Also covered there.
I replaced both battery cables and starter cable (and starter) with the next size larger cable due to a slow crank when hot symptom, a slow crank when hot symptom will kill wires, starter and the solenoid.
This also assumes the engine is not locked up.