Hey Sephy!
First off, "plateau" is a very overused word in the weight loss world. If you haven't lost lbs or inches in 4 weeks or more, that's a plateau. If you're losing either, or it has been less time, your body just might need time to catch up. So patience is the name of the game to start.
As for the challenges, most can just be jumped into. Some challenges have hard and fast signups, like the Biggest Loser challenge, but most in the "Chicks up for a Challenge" area you can just jump into.
Stick around and keep posting - can't wait to hear more from you soon!