If using SQL Server locally, install a Developer Edition of SQL Server. It will make your life much easier.
Make sure that IIS7 is running.
Make sure that the ASP.NET State Server is running.
Make sure that SQL Server is running.
Make sure that .NET version that you are going to use is set in the following the 3 areas:
1) IIS7 Server Level
2) Application Pool for the Website
3) The Web Application in Visual Studio
If using a local instance of SQL Server, setup an Application Pool for your web application with the Local Service account selected.
Add the Local Service account to the security logins of SQL Server and then add the databases that you are going to access to the User Mapping for that account.
Make sure that your connection in the web.config file is using your instance of SQL Server.
Test it out by using the >Start without Debugging command. (This should flush out any errors.)
After all of the issues have been worked out, then use the debugger.