You can enter whatever you want, as long as it doesn't violate someone else's copyright and someone else isn't already registered under it. However, it is a very good idea to register the name legally in some manner (incorporating, creating an LLC, setting up a DBA, or even just start putting SM- for "service mark" - whenever you use the name are all fairly simple to do and will ensure that no one else can use the name later on, when you make it big of course