Whats wrong with my car's electrical system?

i know it is hard to tell without looking at it, but i have a pontiac grand prix GPT. i think there is something wrong with the electrical system in the car. both the back windows don't work, which could be a number of things, the lights flicker, which include all the lights such as the headlights and inside lights. the battery drains easy if something is left on. sometimes the engine is hard to turn over. and sometimes the power will just go out completely like locks and lights. adn then all the sudden it will come back on. i don't know if something is loose or what.. please leave comments and help me out.