By chance - Just keep playing games and meeting new friends. Someone might invite you to join its clan after you played a game with it.
By PM - It is never bad to be proactive. Just ask the clan leaders if they are interested in you. You may get a place. Who knows?
By thread - Open a new thread in the clans forum. Advertise yourself. Ask if any leader wants to take you to a clan. There is always a good chance if you ask.
Clans are people and if you put yourself out there they will recognize you. Don't act immature in games and play in team games the best you can because it shows your skills as a group player.