I'm 14, I weigh in at about 125 pounds. I'm very skinny and have almost no muscle. I have never done any dedicated exercise plan, and I realized that I really needed to have one when I couldn't cut a lawnmower cord with some wire cutters, and the guy next to me cut through it instantly. I don't exactly want to get a lot of bulk or muscle, I just want to be fit and in shape. Can anyone recommend me a plan, and give details of how long it would take?