Let me start by saying that since you are 15 years old and still a minor, you are not going to find a reputable physical coven which will accept you as a member. That's especially true if your parents don't know about your spiritual choice.
That being said, you have the option of joining one of the on-line covens here on SOM if you like. As others have said, just go to the groups page and look at the information about each of the groups, take a look at the profiles of the group leaders, and decide what best suits your interests. And if, after you join, you find that group isn't what you were looking for, you can always change to another group.