Truck just clicks at start won't turn over on 1994 chevy 1500?

Remove and fully charge, or replace, the battery. Clean the battery terminals and cable connections. Examine the cables closely for hidden corrosion and deterioration. Clean all grounds. Afterwards, ensure all connections are tight. Most auto parts stores will charge batteries for free.

If the above good advice doesn't work, I would get a remanufactured Starter, You may just have a bad Starter Solenoid, they cost almost as much as the Starter. The New Starter has the Solenoid arready mounted on.
You could hook a test light on the load side of the solenoid(starter connection) and see if 12 volts is present when you hear the click, this would tell you if it is the solenoid or the starter itself.
Replacement is about $80.