MS SQL Server create user who can access only those databases which they have created How to create and give permission to a user in such a way that they will have full access only to the database(s) they have created not other database(s). I am using SQL Server 2016 Express.
Welcome to the world of SQL Server Security! 

Probably the best place to start is by reading the Microsoft Securing SQL Server Guide. This covers all the basics from securing the physical SQL server to database and user permissions.

An alternative view is the SQL Server Central Stairway to SQL Server Security