When you set up your Mac you entered your full name and created a user name. This is sometimes called a short name since it’s usually a shorter version of your full name or a name you created. It’s easy to forget, even though you’ll eventually need it.
When asked for your user name—for example, in the login window or when connecting for file sharing or screen sharing—you can enter either your full name or user name. The user name is used by your Mac to keep track of all your files and information. Because of this the user name is permanent; you can’t change it without creating a completely new user.
If you can’t remember your user name, open a Finder window and look for the house icon in the sidebar. The house icon identifies your home folder, which has the same name as your user name. If you don’t see the house icon, choose Finder > Preferences, click Sidebar, then select your home folder under Favorites.