The car doesn't start. It does a rapid-fire clicking noise when I turn the key. What could be the cause?

The battery terminals have already been cleaned, so the problem is not a corroded terminal issue. I tried turning on the headlights and they turned on just fine. If the lights were able to turn on, does it mean that the battery simply does not have enough power to start up the car?

I currently have the battery charging. So another question I have is, if the problem is battery-related should I just fully charge the battery or should I get a new battery?