When a car is not starting and is only clicking when turning the key that usually means a few things.
The battery could be low or even dead, you could have loose or bad connections on the battery not allowing it to get the proper voltage or even a bad starter or solenoid on the starter.
First thing I would do is disconnect the battery cables and clean the battery cable connections. You can get a battery post cleaner and you can also dip the battery cable ends in baking soda and water to clean it.
You can also pour some baking soda and water solution on the battery posts itself or brush some on the side terminals if your battery connects on the side.
Also make sure your battery is not low and charged up fully. If you don't have a battery charger you can ask a neighbor or even take the battery out and take it to an autozone and they will charge the battery up for free if the battery is any good.
If the battery checks out and you've cleaned the battery cables and connections and made sure they're tight then I would pull the starter off and get it tested out.
If it's the starter then you'll just need to get a new one which it couldn't hurt too anyway depending on the age of the starter.
In fact if it were me and I took the starter off anyway I would just replace it with a new working starter as preventative maintenance.