What version of SQL Server?
If you are using SQL 2005 on Win7/Windows 2008 or R2
Right click on SQL Server management Studio and Choose "Run as Administrator"
If you are using SQL 2008/R2, make sure your user account (account under you are logged into windows) has appropriate permission to creaet new database. By default in SQL 2008/R2 Windows admins are not given sysadmin permission in SQL server unless othewise
done during the setup process.
refer to Troubleshooting: Connecting to SQL Server When System Administrators Are Locked Out
http://msdn.microsoft.com/en-us/library/dd207004.aspx