Have they convinced you to register against you will?
Here’s what you can do:
Don’t register online! Instead, go to the post office and properly complete two Selective Service registration forms. Write, “I am conscientiously opposed to war” clearly on the front of both forms. Address envelopes to the Selective Service System and to yourself. Both will be postmarked on the same date. The Selective Service won’t pay attention to what you’ve written and you’ll be registered. When you receive the envelope through the mail, don’t open it. Instead keep it with your important things.
If there’s a draft you must refuse. You will be given the opportunity to plead your case in front of your local draft board. Bring the sealed envelope and open it in front of them. Explain that your opposition to war didn’t suddenly materialize when the draft was reinstituted.