The standard signature on Wikia (produced via ~~~~ or ~~~ to provide a signature without the date) consists of a simple link to your user page. This can be customized in your preferences under "user profile." You can create a custom signature by checking the "raw signatures" box, and entering the MediaWiki code for your new signature in the "Nickname" field. If you are using a template for your custom signature (such as {{User:Username/Sig}}), you must enter {{subst:User:Username/Sig}} in your preferences.