There are definitly no one who would advise a teenager on using steroids. You havn't even reached your peak of your own natural testosterone production. You have so much to loose by trying steroids right now.
For example the weight you have gained, and the weight you say you usually gain, is not muscle, or at least 95% not muscle. Most of it is a gain in water weight and fat retention from your increased caloric intake. Yes, As more sugars are stored in the body, water has to be stored along side it.
If you take steroids, I don't think it will give you the lasting muscle/ size gains you are looking for. The reason for this is for one you haven't developed nearly enough androgen receptors to reap the full benifit of the drugs. This comes from years of proper training - the bigger your natty muscle get, the more androgen receptors they contain. That is why someone like me, could gain as much as 25 lbs on his first cycle and keep 15, where as you will be lucky to gain 10 lbs, and keep about 2 in lean muscle gains.
Now the risks you run with steroid use, includes but is not limited to: an increase in acne - I have seen guys your age break out so bad they looked like the calendar boy for the 'before' picture of proactive acne. This acne is very stubborn to get rid of.
Closure prematurely of the epipheseal plates (growth plates) These are caused by estrogen aromatizing (converting) from the steroid you take, and binding with receptors in the ends of your bones to cause your growth to stunt - not a good ideal if your doing this for B-Ball.
Impotency. You wouldn't believe how many guys your age who have done steroids suffer from this. At 17 I could get a boner just thinking about thinking about a girl. Why would you wnat to throw that away? - as it never comes back the same as at your age.
Now of course if you are supceptable to depression, and any of the above sides do happen to you, your going to be in a world of hurt, so please weigh out the pros and cons very seriously.
What you should try first before resorting to steroids, is to enlist the help of a very good personal trainer and diet guru. I can see from what you have said regarding your protein intake, that you are lacking in that area - it should be in the 300 g level. You need to learn when to eat and what to eat, and when to train and how to train. Take the money you would spend on juice and learn instead.
There is a good chance that with your natural hormone levels, and the knowledge of a good trainer and diet guru backing you up, that you could gain those 20lbs sooner than later, and all natural!
Not only
Just kidding around bro, but that's is the truth.