In addition to paradoxed's tips and wise words, here are some other tips and explanation about choosing a magic coven.
Many covens are associated with a specific religion, or at least some sort of spiritual genre (such as being generically neo-pagan). All will tend to ask that you be open to and agreeable with their beliefs, but some covens will not require that you actually be a member of any particular faith. Even those that do not have a set religion will usually have certain beliefs. They may not necessarily be written down or made official, but that makes little difference if every person in the coven believes a certain thing (such as, "vampires do not exist").
Magick, like any power, has certain ethical considerations, and whatever your ethical beliefs may be, I'm sure you don't want to be part of a coven that views your behaviors as evil (and to be frank, I'm sure they don't want you either). Ethics usually relate to any religious or spiritual belief held by the coven, but they are not quite the same thing and it is entirely possible to find two Gardnerian covens with very different views on sexuality or hexing.
How much work does it take to spend time with this coven and how much time can you actually spend with them? I don't know if you want an in-person coven or a coven here on SoM or what, but the same basic principles apply. How often does the coven meet (or post in their forum or whatever), and how often would you like to meet (or discuss) with your coveners? If you only want to meet every few weeks you'll miss out in a very active group, and if you want to meet frequently you will feel very unfulfilled with a group that meets only on full moons. Also think about how hard it is to actually meet with them.
We know you want a magically active coven, but in what way? Do you want big group rituals, or small workshops on specific things? Or would you like classes on solitary magick? Or perhaps you seek a coven that likes to practice spellcasting upon members. And what about other things covens do? What about things which are not magick, but which are magick related (like yoga and meditation)? And would you prefer one type of magick over another (New age magick over Hermeticism, for example)?
I really hope this helps you. As paradox said and very well, observe all of the online covens and decide which one fits you by following these tips.