Restricted user on my local SQL Server database?
If the instance name is .\SQLEXPRESS you have an instance of SQL Server Express but not localdb. 

The default install will give builtin\administrators permissions on the database so you will have to start the Managment Studio (SSMS) by right clicking the shortcut and select run as admin to have an administrative token and be able to create databases. 

When running the SSMS as admin you can add your user account to the sysadmin role in the database server and after that run things using your user account without elevation.