A Mediator is a Trusted trader who has offered to help people with trades, and nothing more. Please remember these are trusted people who are offering their time to help you with your trades, and therefor it is recommended that you are very appreciative of their generous service.
In order to become a Mediator, you must first obtain the Trusted status. After that you can PM any Moderator and tell them that you wish to help as a Mediator.
Please read the help topic How To Request A Mediator And Prevent Scams
for additional information.