There's three totally different symptoms. One is a light, barely audible, single click each time you turn the ignition switch to "crank". One is a pretty loud single clunk, and one is a rapid succession of loud clicks that sounds like a fast machine gun.
If you get a single, loud clunk each time you turn the ignition switch to "crank", that is caused by worn contacts inside the starter solenoid. Many hardware stores and farm supply stores have the repair kits for that. They cost around $12.00. The clue to this problem is the starter will work eventually if you keep on trying it. As the contacts wear it will take more and more attempts to start the engine.
If you're getting multiple clicks, while you hold the ignition switch in the "crank" position, that suggests a run down or bad battery, or a bad connection on one of the battery cables. You'll need to measure the battery voltage to start that diagnosis.