How do i correctly paint my room for the best look?

So this summer i decided to re-do my room to make it more organized for high school but you see my father thinks he such a handy man when he has no clue in what he is doing and eventually just messes it up sometimes when he does know how to do something he does a very crappy job of it. we try to tell him but he just so stubborn. So he is going to do somethings in my room i am not sure but it has something to do with replacing old rotting wood in the frame? or something am not so sure but then we are going to paint my room this lovely color i picked out and i then realized my father does not know how to paint! my proof i am sitting in a room with several roller marks and white spots he missed and did not know how to cover. so i need to get informed on what i need what i should do step by step to make my room remarkable!! You should know there it one gallon of paint are we supposed to double coat? Another thing i hear you need primer it this necessary? There is one other coat of paint underneath the current color now. i really would appreciate it for some great tips please and thank you. :D