Every now and then they ask for moderators. To express interest keep using the site. Help out with whatever you can at the reputation level you're at. If that means flagging junk, voting to close things, editing things into better shape, deleting things that need to be deleted, do that and keep doing that

@tjt263 there's two ways moderators are selected. For new sites, they ask for people to volunteer themselves to be mods, who are selected. For older ones, its on an 'as is' basis. That said, a high rep user has many of the powers we have, other than the really dangerous ones.