Here I already explained how to debug IIS express and
here I explained how to add web site to IIS.
Now I am will give little bit more detailed explanation how to debug ASP.NET on IIS.
First start Visual Studio as administrator:
Set breakpoints and attach to process w3wp.exe:
If you don't see w3wp.exe then either refresh browser, or open your application in browser, and click refresh in "Attach to process" window.