when you're trying to pick a coven, the most important thing is to think of what you want to learn. once you figure that out, its easier to find an online coven, as for a physical coven i haven't figured that one out yet, but if you figure out where to find one let me know
If you are talking about the Spells of Magic online covens, then I have advice.
Firstly, the only person who can decide what's a good coven to be in is you. I cannot tell you because I don't know you like you know yourself. Find whichever one suits your interests, and then send an application via mail to the Priest(ess) of that coven, and click the "Apply To Coven" which should be on the coven's main page.
Secondly, you have to be Council position or higher in a coven to post spells.
As an addition to the fabulous advice given in earlier comments if you are looking at covens and aren't sure if what you read will exactly apply to your fancy, e-mail the leaders. As long as you are polite and remember that you are inquiring about something that may be personal so as to be treated with utmost respect and sincerity, I'm sure active leaders won't have a problem answering question about the specifics of what they teach, offer, and provide as a coven.