Go to the Moderator Center in HQ. You will see all the Niches looking for Moderators. Find the Niche(s) you would like to Moderate. Click Election, and nominate yourself.
Another way to do this is to Search for the Niche, when the Niche page pops up, click on Moderator Election. The will bring you directly to the nomination page.
Here is a short video to help you with either way.