The clicking sound is usually an indication of a poor connection. It might be the key switch, the starter solenoid, or the wiring. It can also be a combination of many things causing high resistance in the wiring. Clean the terminals on the battery and if that resolves the problem you are set. If the problem persists, then continue down stream cleaning and checking for tightness all the connections. If the problem persists after you have checked all the wiring, then I would suspect either the solenoid or the ignition switch. To check the ignition switch, when you get the click, hold the key to the start position and then try to wiggle the key to see if that gets the starter going. If it does, then it is the switch. If not, then I would check the solenoid, starter brushes, or starter armature end bushings for wear.