To create a Den, you must not already be in a Den. Tap the Den UI Button in the HUD, and from there you'll be able to search for a Den to join, or you can create your own for the price of 500 coins.

You will be able to customize the Den's Name, Description, Type, Language, Minimum Dragon Power, and a custom Emblem.

You can also mark a Den as "Private" which will prevent it from showing up in search results unless someone searches with your Den's exact name.