I assume you know how to find the Clam page that lists all the clans? 4th tab in on top of the page.
What type of play are you looking for? Your best bet is the social clans devoted to a certain style. Anyone can join them and you just bug the other players for games. People are usually pretty good at helping others.
Borg: Team games only
Real Time Social: devoted to real time games
Duellers Socitey: 1v1 games
Most battle clans are already full and are usually invite only. But who know? Might as well hit them up. They are Whiners and Crybabies, Cosa Nostra, League of Shadows, Old Soldiers (age requirement also). Not many battles going on right now though.
to get an invite to a social clan...
1. go to clan tab and click on a clan, you are taken to their page
2. on right side of page, send an email to clan admin via "comments" and "send note to clan admin"
as a heads up, clans are what the members make fo them. if you want games out if it and to learn, you have to be proactive. Asking in thsi thread is a good start.