A car that only clicks when you turn the key to try and start it can be caused by several things.
But the most common reason a car is clicking and not starting is due to a low battery or loose battery cable connection.
Try charging the battery up first and if it still won't start then check the battery cable connections to make sure they're tight and also make sure the battery cable connections are not corroded.
It's a good idea to remove the battery cables and use a battery post terminal cleaner to clean the battery posts and battery terminals up anyway so you're sure the battery and cables have a tight secure connection.
If the car still only clicks and doesn't start then another problem could be a loose battery cable connection to the starter or solenoid or the solenoid or starter could be bad as well.
If everything else checks out and your battery is fully charged up and all cables are tight and you still only get a clicking noise when you turn the key to start your car then you need to have the starter replaced or solenoid replaced.
However most new cars today have the solenoid mounted on the starter and the starter has to be removed anyway so it's usually just as easy and cheap to replace the whole starter and solenoid together.