1998 Mitsubishi Lancer battery keeps dying

Hi there, something keeps causing my battery todie in my lancer, iv changed the battery twice so far its only a month old. im not what it could be ? any ideas ? maybe the alternator? Or maybe some kind of electrical problem ? Iv had an amp and subwoofer hooked up , but unpluged it as i thought thats whats causing it to die, but it wasnt, and i never lock the car with the alarm any more so that cant be causing it to die either .. omg what could it be? please help me out . im stuck with no idea of what is wrong with it. i thought id ask you first before i take it to an auto electricians who would charge big fees just to have a look at it . please help . cheers bree