My F150 only clicks when I try to start it.

Find the starter solenoid and try to read the voltage on it when someone turns the key. it could be a starter or thr solenoid which are both do it yourself repairs with correct tools.

A low battery sometimes has enough amperage to burn relay contacts but not enough to make it work. It's possible that the relay has failed.Starter can also be affected in the same way. Also check battery cables and make sure power is getting to cable and not being stopped by corrosion inside the ends.Use a volt meter and check power...it will tell you where it it drops or stops.