This isn't about how to find a coven, or how to start one. Just simply some things to think about if you are considering joining one. Some traditionalists say that you must be initiated into a coven in order to call yourself "Wiccan", but I prefer to take a less-formal approach and let each person decide for themselves.
Good Reasons to Join
You're interested in a tradition such as Gardnerian or Alexandrian. The beliefs and rituals of both these traditions are passed down from one coven to the next and typically not shared with outsiders. In order to truly understand what they are all about, you must be properly initiated.
You're looking to expand your horizons. There is certainly nothing wrong with doing ritual on your own, but it is a whole new experience doing ritual in a group. Being in a coven can teach you a lot about your own spirituality.
You need some structure. Face it, some people just don't get anything done unless there is some external motivation. Being in a group can help you keep your studies and spiritual growth on track.
You just like being part of a group. Group rituals raise more energy, and it can be lonely as a solitary.
Poor Reasons to Join
You feel you have to. Though some traditions to require coven membership, you can happily follow a Wiccan path without belonging to any group.
You want to have fun. Well, covens most certainly have fun together, and some covens are less formal than others; but it is a serious commitment to join a group and you should take it seriously. If you just want a social experience, I suggest finding a moot in your area instead.
You're only interested in scholarly study. Study and learning are a big part of coven life, but there is an emotional component as well. As time passes, most coven members grow together like a family. If you do not want that kind of bond, you may be better off finding just a study group or take some classes.
Please note these are not hard and fast rules by any means. Just some things to think about.
If you decide that joining a coven is right for you, make sure to do some research! Talk to people, ask lots of questions and go with your gut. If something seems 'off' about a group of people, then keep looking.