Open a challenge and click "Join" — for most challenge types this is instant and just adds you as a participant. Some challenges require you to accept a legal/IP agreement as part of joining; if so, you'll see that step before your join is confirmed.
Joining doesn't commit you to submitting — you can join, look around, and decide not to submit with no penalty. What matters for judging and prizes is only whether you actually submit before the deadline.
