Most guilds which are successful and established already have reliable tanks. What you can do is find one that has a social invite policy or needs benchwarmers and use the tank ability "Patience". Be available for raid time, have your rep farmed, gear properly maxed out with gems and enchants, etc. Also very good to start guild groups for random heroics and LFRs and make some friends while you're at it.
It would also behoove you to make sure your dps spec/set is up to snuff.
It is not going to be fast or easy, so, good luck.
"There are no men like me...only me."