The best place to look for covens in your area is at http://www.witchvox.net They have listings for covens by state and then by city all over the United States, and by country all over the world. Check out the column on the left hand side of the page for links to listings of individuals, groups, events, and shops.
If you don't see a coven listed but you do see listings for metaphysical shops or you know of a metaphysical shop in your town check there as well. Not all covens advertise locally but might be known to shop owners in their town.
If you are under 18 it is unlikely that any reputable coven here in the United States will accept you as a member because of legal issues.
Not every community has a coven and not every coven is reputable. Always check out local covens carefully before joining one. Or, if there isn't one in your town you might need to consider travelling a bit to belong to a coven that is a good fit for you.