My car won't start even with a new battery?

It is most likely your alternator- the device that charges your battery when your car is running. So if your alternator isn't working your battery won't charge when you are driving, so even if you charged it previously it will run out of power again. It will most likely need to be replaced, but when you take it to the mechanic don't mention that you think you know what it is- it is a rather expensive part, and if you mention it they will just replace it willy-nilly and charge you an arm and a leg. It could be something else cheaper, but I doubt it.