The solenoid has two battery cables on it, one to the positive of the battery and the other to the starter. There is also a little wire on it that when electricity is supplied to it it should connect the two cables together internally in the solenoid. The little wire comes from the key switch through safety switches like on the seat and clutch brake pedal . A cheap quick test is to take a short piece of wire and touch the cable coming from the battery to the little wire terminal on the solenoid if it makes the starter work it is probably a safety switch or key switch issue not a bad solenoid or starter.

Unhook the wires and clean with sandpaper. un bolt solenoid and sand where it grounds. if this don't work replace solenoid.