Being on the site for 3 months is not enough to get you a leadership position. You need to have made an effort toward contributing a fair amount of knowledgeable information to the site. (Threads, articles, etc)
At the bottom of the covens page, there should be an area that lists the inactive covens. Next to each there will be an option that allows you to apply for leadership to one of them.
All applications are looked over and the best candidate for the job is picked and promoted, thus, there is no guarantee that you will be placed as leader of the coven.