To change the URL you need to contact the Wikia staff. Since you don't have many pages yet it might be just as easy to start up a separate one and leave the de. one there in case you want to continue on with a German version.
Having both is pretty easy. Go to the Create New Wiki page, make sure the language is set to English, put in teentop, and it will create a teentop.wikia.com that's separate from de.teentop.wikia.com. I guess it doesn't add the en. since English is considered Wikia's default language.