When you turn the key to your cars ignition and it just clicks then the clicking and not starting could be due to several different problems.
Most common cause though of a car that clicks but doesn't start is loose or corroded battery cable connections on the battery terminals.
The battery terminals could be corroded as well. You can buy a battery post cleaner at auto parts stores and then remove the battery cables and clean the terminals and battery posts.
Another good way of cleaning battery terminals is to soak the battery terminals in some baking soda and even pour baking soda on the battery and then wash it off with water.
If you've cleaned the battery post and battery terminals then you may need to make sure the wires are making contact in the battery terminals as that's another common problem of a car clicking and not starting.
If that doesn't fix the problem and your battery is fully charged and not dead then it could be the start itself or the starters solenoid.
But also check the battery cable connections to the starter too to check for loose wires.
A stuck and failed starter can cause your car to just click when trying to start so you may need to replace the starter if all the above doesn't fix it.