Every logged-in user has a user page. (For example, if your login name is "MyNick", your user page will have the title "User:MyNick".) You can find your user page with the link at the top right of every page, or by following this link. Most people use user pages to give information about themselves and their interests and keep links to useful things.
You can make your user page as simple or as complicated as you like. You can add formatting or colour with wiki markup or with HTML, or leave it plain and simple. You can also add pictures, either a picture of yourself, one that expresses something about you, or just one you like. However, all pictures added to The One Wiki to Rule Them All must be free-use.
Be aware that all pages on this wiki can be picked up by Google and other search engines, and are available under the GFDL, so keep that in mind when you write.
You may not use your user page for any content that is prohibited on Wikia. This includes copyright violations, pornography, excessive advertising, promotion of illegal activities, and hate speech. See the prohibited content page on Central for more detail on this.
Other policies for what you can and can't have on your user page should be by community decision, and will differ on every Wikia. However, most communities allow users to do whatever they would like with their userpages, within Wikia policy.
Editing user pages
User pages are still wiki pages, so they are open for everyone to edit. Most people will ask before making major changes to someone else's user page though. Small changes, like formatting fixes or spelling corrections, are absolutely fine, and can be done freely - it's always nice to find your user page improved.