If you go to groups, there's a list of covens that you can choose from once you read about them. Spell Casters is an automatic coven so once you go to the top page after clicking on it, there will be a link that says "Accepting Applications." Click on that.
Other covens require you to mail the high priest or priestess which is listed at the bottom of the coven page you want to join. They should have an application list to where you need to answer questions and submit it to them through mail. If you're accepted they'll tell you and once again you can go to "Accepting Applications" and join.
Hope this helps, best of luck!
Dyna , my suggestion to you is to view all the coven's profiles, and decide which one you want to be a member of the most. Follow the appropriate procedures for that coven, and stick with it until you are accepted or rejected. If there is a coven rule based on age, mail the priest and/or priestess with this to start with, and ask them if that would bar them from the coven. Chances are, if you are a quality member, they may choose to make an exception.
Don't jump into a coven, just because it's easy to join. You'll likely end up somewhere you are uncomfortable.
Caraway , not all require the priest or priestess to be mailed. For isntance my coven, Teen Witches, does not require mail of any sort, however I consider a mail to me to be an indication of how active a person will be. I and Greg go through the applicants' profiles one by one, determining whether to accept or reject a person based on their profile picture and biography (or lack thereof), as well as their history in the cases of those we are unsure of.
There are several other covens that work through a different dynamic, where the whole of the coven votes upon who will join based upon the application sent to the priest and priestess.
Most priests and priestesses will also want you to put in both the mail application and the application that actually allows them to accept you (under that Accepting Applications link) at the same time. They prefer this so that they can quickly and easily accept you once a decision has been made.