When you create a board, you automatically become its administrator and, accordingly, its first moderator. You can then add other users as moderators and do other cool stuff. Visit the boar management page at the web version of the platform.
If you want to moderate an existing board, then you need to ask the board administrator to add you. If existing moderators need help and you prove that you can be useful, they can add you to the moderators.
In cases where the board administrator and moderators are not active, you can ask the platform administrators to make you a moderator by publishing a post in b/moderators.