Not really something SharePoint related but from time to time I need to create database schemes and therefore I use MS Visio. MS Visio is very easy to use and with the Forward engineer plugin you can generate SQL output to create your tables.
When I was designing my database structure I could not find the option to set an identity column. You can find this option in the Data Type properties:
Select the datatype (int) and click “Edit”.
Click the “Edit” button.
Select “Identity” and click “Ok” twice.