my car acts dead all the time HELP!!!!

in my car i have to turn the key back and press the button to make it go completely off. well sometimes when i do that and i go to start my car it acts dead.the lights on the dash do come on though. but itll start 20 mins later.. then some other times it will just be completely dead and ill need to jump start it.. can someone tell me whats wrong.. i think its my altenator but i wanna know for sure before i go blow money on one and it not be that.. i think it may be that or my starter is pullin too many volts when i start it.. PLEASE HELP!!!!