don't worry about pull ups just yet, gotta agree with John with the press ups, you'd best start with them and then move onto weights later on, I'd recommend you make the transition to weights once you can do 40 press ups with good form and 60 sit ups, i'd also recommend squats as well but if you're overweight you should be doing a bit of cardio (running, swimming, walking etc.) Cycling's good if you can be bothered to wake up earlier in the morning to get to school/work.