How to create a User Account in Microsoft SQL Databse

It's a two step process.

First you have to create a login to the instance.

That can be a SQL Server account (with a password) or you take take the Windows AD login and pass that through.

Then you create a user in the database connected to that login and grant appropriate privileges.

use master
go
create login [fred] from windows with default database=[master]
go
use demo
go
create user fred for login fred
go
grant select on table to fred

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The user name and the login name do not have to be the same.  Make sure you grant object permissions to the user, not the login.

There are exceptions, for example you can make a login a system wide administrator for all databases on the instance.

From the documentation:

Creating a login: http://msdn.microsoft.com/en-us/library/aa337562.aspx
Creating a user: http://msdn.microsoft.com/en-us/library/ms173463.aspx

you can also right click on the database, and use the graphic user interface

If my domain is MYDOMAIN and my account is sjwales it would be:

create login [MYDOMAIN\sjwales] from windows with default database=[master]

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All of this can be done from the SSMS GUI as well as indicated by esskayb2d.

The username could then be anything, drop the domain from user if you want, for example.

create user sjwales for login [MYDOMAIN\sjwales]

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and then assign appropriate permissions to sjwales as needed.