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.