I want to become a game moderator but I don't know how to

You should always start by asking the super mods of the game through their social media as they are able to add mods. Also try the game's forum, many users get notifications when threads are created for games they follow.

It tends to be a "don't call us, we'll call you" kind of thing. Be a positive part of the game's community. If more mods are needed and you're someone the community trusts and appreciates, that'll give you an excellent chance. It's rare in my experience for a game to need more mods, though, unless all the existing mods abandoned it/SRcom altogether.