Sorry, I assumed some mechanical knowledge on your part.
Here's the thing, there are two solenoids.
One is on the engine harness and one is right on the top of the starter.
There is a solenoid mounted on the engine which engages the solenoid mounted on
Either one could be the problem, or the starter motor itself, we need to figure out which it is.
The easiest way to determine if it's a solenoid or the motor is to jump power directly to the motor, if the motor is good, it will spin.
If it doesn't spin, the motor is no good, if it spins, one of the solenoids is no good.
If you aren't understanding this, shoot me some more questions.
If the motor spins, we need to figure out which of the solenoids is bad.
Look up "slave solenoid" and do a little reading and get back to me, please.