You cannot become  a merit source. This is a position that comes with a lot of responsibility and thus you need to be an established member here to apply for this. Admin has not defined any fixed criteria for being this established member but I think he meant that the profile should be at least an year old and should have made some quality contribution here. You can work on both aspects and apply later. To apply, for this a member has to quote 10 posts from the forum (of other members)  that he thinks have not got sufficient merits.