check the voltate at the S terminal on the starter
it should be battery volts...
if it is anything less than battery volts when you turn ignition to 'start mode' then check for resistance in the circuit.

you could try using a jumper wire straight from the battery to the S terminal and see if the starter will turn over then, if it does then you have a problem in the control circuit. If it does not then the problem is in the starter solenoid.

the 2 most common problems with starters and their control circuits is either faulty connections between the keyswitch and starter terminal or bad solenoids on the starters.

But before you even do this make sure the battery is fully charged, that can also be a major factor in a starter failing.