Why do you think you have arthritis?
Arth-itis is the medical term for inflammation of the joints. While that sounds like something you might think you have, it is really a disease for the elderly and the frail. You are 17 years old and physically fit, by your own account. It is so unlikely that you have congenital AI with rheumatic symptoms only coming on now, that I'm willing to bet that (barring any tests which you might have had and not mentioned) you don't have arthritis.
But let's just say you did. You don't, but if you DID, the answer would be YES. You can continue to work out. In fact, you SHOULD continue to work out. Lift as heavy as you always have until the day you can't.
And then lift lighter with more reps.
The second part of your question, which makes me wish I hadn't bothered with the first part, is NO. You can't get either for arthritis, and if you ask for it, they put your name down as a drug seeker. Then you really are f@@@ed.
And a little Wisdom = Steroids make you weak. They make you impotent. They make your muscles grow and your pecker shrink. You can look good for the women, but you can't do anything with them. You can sculpt, but you can't get strong. And the second you stop shooting them....you go flabby and worthless. Your heart muscle becomes like a floppy balloon, and won't pump right anymore. You get congestive heart failure in your 30's. Your adrenal glands will be shot. You'll be weak and pathetic and sickly. And you'll WISH arthritis was your worst problem. I've seen it happen. It's ugly.
Don't listen to me as a doctor. Listen to me as the man who's world power lifting record still stands, and has the room full of trophies to back him up. Be strong or be stupid. You can't be both.