When installing SQL Express, you’ll usually get a named instance called SQLExpress, which as others have said you can connect to with localhost\SQLExpress.
If you’re looking to get a ‘default’ instance, which doesn’t have a name, you can do that as well. If you put MSSQLServer as the name when installing, it will create a default instance which you can connect to by just specifying ‘localhost’.
See here for details… MSDN Instance Configuration