go to the page for them (like hover over "my home" click on tournaments, and then browse through up coming tournaments and check 'em out).
Then join one you want.
A word of warning, a tournament here or there can turn into an overwhelming # of games. Some people dig an overwhelming # of games, some don't. I'm currently in way over my head and it might only get worse for me with regard to # of games.
OK how do you play in a tournament?