Shit, that's... 3 different threads in one. I'll do my best to answer you succinctly:
If you don't jog already, start. Muscles don't mean jackshit if you have absolutely no endurance. Do you have access to weights? Either at school, home, local gym, etc.? If so, get into it, but very lightly at first. I would give you a routine, but #1. it's hard to explain in just words the correct way to do each exercise, and #2. I don't anything about your body. My advice is to go talk to the coach of the team, or a personal trainer. Get a workout tailored to your needs.
I suppose one thing you should improve is your footwork. Go out in your backyard, local park, whatever, and set up a course for yourself. Put a bunch of cones in two lines, each about 10 yards apart, and sprint back and forth from one cone to the next. This will work on your agility, footing, and quickness. For overall speed, nothing beats sprints. Like I said before, though, none of this is worth anything if you have no endurance, so build that up over time.
Finally, for gaining weight/getting buff, do like I said about lifting weights. Also, eat more lean protein (egg whites, chicken, fish, shit like that) in your diet, and if you're really dedicated, go get some whey protein powder and have a protein shake after each workout.
That's as succinct as I can be, you really asked questions that I could write a dissertation on.