I was at a gas station and turned the car off

I was at a gas station and turned the car off. When I tried to turn it back on it would try to turn over but wouldn't start. I finally got it started but it died immediately. Tried a few more times and then it wouldn't even try to turn over it was just clicking. My key wouldn't turn back all the way so I had to leave my car with the lights on therefore it completely killed my battery. I took my battery to auto zone to have it charged when I put it back in the lights came on but when I tried to start it it was still just clicking from the fuse box it wouldn't try to turn over. Could this still be my battery if I bought a new one? Or does it sound like the ignition switch or the starter. Or even the relay switch. I want to buy a new battery but if that's not it I don't know what to do.