I will assume that you are referring to online covens here.
If you want to join a coven, read the coven page first. Some covens want that the member fills out an application and send it to the Priest/ess, otherwise you won't be accepted as a member, even if you click the join button.
First, you need to click on "Accepting Applications" present on the coven page, then click on Apply. Then, mail the respective Priest or Priestess of that coven with the application, stating there the answers of the questions of the application, which you will find in the coven page. After that, all you need to do is wait.
If the Priest/ess finds you suitable for the coven, then you will be accepted as a member. But if they refuse to accept you, then find some other coven and do not argue.