If you are asking how to make your own coven here on SOM the answer is that you cannot. The Site Administrator has determined that there are currently enough covens here and has disabled the ability to create new ones.
You could try to take over an inactive coven. You need to be a member of the site for approximately 3 months in order to apply, or to be promoted by a current coven leader. Being on the site for 3 months is not enough to get you a leadership position though, if you are going about it via the application process. You need to have made an effort toward contributing a fair amount of knowledgeable information to the site through threads, articles, etc. You also need to have shown yourself to be mature and responsible.
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. You only get one application, however. Once you apply to lead one coven, you cannot apply to lead another.
All applications are looked over by the site Admin and the best candidate for the job is picked and promoted.