If that's a rapid ticking you're hearing, that is a sign of low voltage caused by either a defective battery or loose or dirty cable connections. Watch what happens to the brightness of the head lights or dome light when you try to crank the engine. If they get dim and flicker, measure the battery voltage right on the posts, then again on the cable clamps while a helper turns the ignition switch to "crank" for you. The voltage before you crank the engine must be near 12.6 volts. If it's closer to 11.0 volts, have the battery tested.
During cranking, the voltage on the cable clamps, (and the posts), must stay above 9.6 volts. If it is lower than that, one or both of the cable clamps is loose or there is corrosion in one of the connections.
If you're hearing a single clunk each time you turn the ignition switch to crank, that is a sign of worn solenoid contacts. They can be replaced for 20 bucks but most people just replace the entire starter. It always starts out being intermittent and will crank eventually if you try it enough times.