i wouldnt recommend it because like you said you are still growing. id wait until you got in high school.

umm yeah it's fine but stay aways from squads and heavy shoulder press

honestly I am a bodybuilder, 20yrs old, 5'11, 259 @ 10% bodyfat and will be competing next year to earn my pro card. The "myth" you hear about it stunting your growth is absolutely bogus, dont believe me? go ask an endocrinologist. I began lifting at 13yrs old and both of my parents are 5'7 but yet im 5'11 1/2". Your body from ages 12-22 are when natural testosterone levels and growth hormone levels are at their highest which is ideal for muscle growth. I will give you the best advice that money can buy. Train each muscle once a week, eat high calories(not junk), do cardio, and never forget your goals. For someone your age here's how I would do it:

Monday/Thursday: Chest + Biceps + Triceps
Bench Press 3sets X 10repetitions
Dumbbell Flys 3 X 10
Pushups 3 X as many as you can
Dumbbell Curls 3 X 8
EZ Bar Curls 3 X 8
Dips 3 X bodyweight

Tuesday/Friday: Back + Shoulders + Legs
Lat Pull-downs 3 X 10
Seated Cable Rows 3 X 8
Dumbbell Shoulder Press 3 X 10
Shrugs 3 X 10
Leg Press 3 X 10
Calve Raises 3 X 10

***make sure you do some crunches on all lifting days to develop a solid core, as well as doing 30minutes of cardio after your workout on lifting days***

The keys to building quality muscle are EATING, TRAINING HARD, AND SLEEP. Don't fall a victim to steroids in high school as they are usually readily availible, that is my honest advise to you because you have no idea how badly it will mess with your body in the long run if taken at an improper age. It's great to see kids like you interested in bodybuilding. If you really want to gather a great amount of information go to bodybuilding.com and go to the forum section. It offers everything imaginable. GOOD LUCK!