It's easy, you just scroll down the page and look for "Tournaments" forum. Check one of the active tournaments, and if the Sign-up is still open then sign-up for the event by filling the official form we use when signing-up for a tournament.
We host new tournament each 10 days. If all tournaments have already started then wait for 10 days for the other ones. We host on the 1st, 11th, and 21st of each month.