first, and on a related note, i want to let you know, there are only a very small percent of bodybuilders that will ever become pro and only a small perfect of these pros will ever be successful. there is nothing wrong with making this your goal, but if you love fitness and bodybuilding, i would focus on building a career around it at the same time. remember that most pros have day jobs and have to pay for their own contest prep. it's very expensive, so if you want to make it work, you should really focus your effort on finding a way to make $$ doing what you love.
now, on to your question. i think that in this point of the game, taking steroids would be counter productive. the health risks for a guy your age are a lot higher than they are for mature adults. there is a chance that steroids could prematurely close your growth plates, stunting your overal frame size. even if you aren't going to grow taller, your long bones will continue to grow for several years. these are bones like the clavicles. they will determine your shoulder width. stop them now, and you may never have a shoulder span as wide as you could have. right now, and over the next several years, you are producing the most natural hormone you ever will in your life!!! take advantage of that. if you begin training hard now and learning everything you can, and most inportantly, STAY CONSISTENT, you will have a huge advantage over your competition. it take years to learn what works for your individual body...what training and diet you best respond to. and this is the foundation of all that is bodybuilding. you'd be shocked to see how many people i come across who are using crazy amounts of steroids and you can hardly tell they're "on". its because they never got a solid foundation and never learned their body. they think that a magic pill will get them where they want to be but they neve put the work in. this sport is 100%, all about hard work. if you are willing to put the work in, all the time, if you want to or not, and you truely love the sport, you will eventually be a success....pro or no pro. this is a promise. just remember that this sport takes a long time. it will take years, even with steroids. if you get the foundation now, when you turn about 21 and if, at that time, you dicide that you want to go on gear, your gains will be shocking. if you want to be pro...if you want to be a monster. its going to be a long term commitment. follow this outline, and when the time comes where gear is a safer option, you'll do very very well on it.
best of luck to you, buddy....now get to work on that diet and training!! theres always more to learn.