Really though the most important part is the contributing and being helpful. That's the number one thing mods look for when it comes time to appoint someone new, "who's been helpful in <forum that needs mod>". It turns out the design is kinda clever because no one has the stamina to post continuously in a forum if they're just looking for a modship.
Anyway, there it is, how you or anyone else can become a mod.
If you really want to be a mod (you don't) start posting in some section regularly. Do everything you can to be helpful. Don't talk shit on the mods/admins. That doesn't mean brown nosing, that just means not yelling "mod abuse" every time you don't agree with a decision and keeping a cool head in S&S. If you really want to be a mod, don't pretend like you don't, but also don't go asking to be made a mod, it doesn't help your case at all (see STD). If you want to become a mod sooner rather than later contribute to sections where there are inactive mods or not many mods. Don't try to become a mod of S&S, GS, BI, or LL&R, those spots are generally picked from the existing pool of mods. I also wouldn't try for BC, but that's just because I have no idea what the dynamic is like there and I don't see it getting any new mods any time soon. LOLI, Music!, G&G, N&N, Math Science & Engineering would all be good ones IMO, assuming you're qualified to post constructively in them.