Instructions
Play at least for a few weeks on the server on which you want to become an admin. Most server admins are reluctant to give admin privileges to someone they don't know.
Behave in a mature and friendly way while playing on the server. When they are looking for new admins, current administrators tend to choose people who have demonstrated that they could be trusted and that they could keep their cool even in tense situations, such as when tensions develop between players.
Visit the server's homepage and look for any page or forum thread containing guidelines on how to become an admin on that particular server. Each server has its own rules and its own way to apply for admin privileges.
Apply to become an admin. If the server's website contains guidelines on how to apply to become an admin, follow the procedure. If the server doesn't contain such information, simply send a private message to a current admin. If you want to do it while in-game, you can press the "C" key on your keyboard to open the chat window and talk to the admin.