SQL Server master user privilege problem

Hello All,

I have created an SQL Server - Enterprise Edition instance with engine version 11.00.5058.0.v1. After the creation, I am trying to perform some SQL operations but I run into permission issues.

CREATE LOGIN userName WITH PASSWORD = 'sameFakePassword'; ==> SUCCESS
CREATE USER userName FOR LOGIN userName WITH DEFAULT_SCHEMA=dbo ==> Error : User does not have permission to perform this action.
select current_user ==> guest

I am not so familiar with SQL Server, but is it normal to see the current user as guest, although I used the master username and password to connect to the database.

Thanks a lot!