Clans are all based in this forum: http://forum.toribash.com/forumdisplay.php?f=97
If you wish to join one, simply follow the application procedure for that particular clan.
But, I would strongly suggest that you start off applying for this clan: http://forum.toribash.com/forumdisplay.php?f=532 As they help out beginners and teach them how to both play the game, and navigate the forums.