If you are asking how to diagram an existing database, from within SQL Server Management Studio open the database, and select Diagrams. Add your tables, and it will create the diagram for you.
You can also use Visio to reverse engineer your database.
If what you are asking is how to create the database in Sql Server from the ER diagram you designed, take a look at the following article:
<a href=”http://www.informit.com/guides/content.aspx?g=sqlserver&seqNum=69″>Database Design: Creating The Physical Database</a>