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There's nothing set in stone as to how you become a mod, but the basics are:
1. Be an active contributor to the forums you post in.
2. Be respectful (Don't troll or spam or anything like that).
3. You usually have to have a clean history (No bans or infractions, at least in the recent years)
Other than that, it's up to need of moderators in the forums you contribute to. If they need a mod, then current mods will typically vote on some candidates and ask around to see if anyone has any objections to your promotion to moderator. A lot of times you have to start as a TM.
If you have any more questions about it you can also ask Sports Illustrator. He's an admin.