For starters, you need a volt meter and need to check the battery voltage as Roger suggested.
You really want to see 12.4v-12.8v. If you only have 12.0-12.2v, your battery is dead.
Second, once you confirm 12.4v+ at the battery, follow the heavy cable to the starter. See if you are getting 12.4v there as well. If you are, push the starter button and see how much that voltage drops. It shouldnt drop down farther than 11-11.5v. IF it nose dives down to like 8 or 9v, battery is dead.
IF all the above voltage tests are okay, then, then try jumping the solenoid on the starter. MAKE SURE ITS OUT OF GEAR. See if the starter spins then. If it does not, and you are positive you had good battery readings mentioned above, you have a starter problem. They "can" be rebuilt, or have brushes changed, but sometimes it is just easier to throw a new one on there.
Also, are you SURE that a click is all you are getting? Lay a hand on the starter and push the button till you get the "click". See if the starter is actually spinning but not cranking the motor. 12v conversions are hard on the bendix of the starter, and overtime with the harder engaging, the bendix can break off the nose of the starter, and then it will spin all day long but not crank the motor.
