Why won't my car start? I put a charged battery in it.. it's not the alternator

Thanks man. Yea I found a better battery and it started up. Then I turned it off for awhile. It would not start back up. So I took a broom handle and knocked on the starter solenoid and, wouldn't u know it, it started right up and it starts up every time.. but I do have another question if u don't mind lending some more of ur knowledge on a young man like myself. My gas gauge always reads empty so I have no clue how much gas I have. I looked it up but they just say it's either a fuse or ground wire, the flex fuel module, or the entire sending unit piece that'll cost me an arm and a leg just to purchase, let alone installation.. any ideas or suggestions.. please help me out. It really sucks, not knowing how much gas I have.. hope to hear from u. Thanks again