How do you create a category/become a mod of a category?

Moderators don't moderate categories, they moderate the entire game. Moderators of games have the power to add or remove categories to any game. Super Moderators have the ability to add other users as moderators to the game. Moderators will typically add users who are available, trustworthy, knowledgeable in the game and maintaining a leaderboard, and have some involvement in the community.

If you'd like to request a category to be added, find a moderator of the game and ask them about it.