Please go to the Tournaments section to choose the tournament you’d like to play in or just scroll down to the starting page of FOA and click on the tournament of your choice. Then click the "Join" button. You will be notified by email 30 minutes before the tournament starts.

If you can't join a tournament, it might be that the maximum number of participants has already registered, or the tournament is being held for a specific group (groups can be specified by rating or region).