There are two level of asp.net debugging

1. Page level debugging
For this we have to edit the page level debugging enable the trace to true in the line in the html format of the page.

%@ Page Language=”vb” trace=”true”AutoEventWireup=”false” Codebehind=”WebForm1.aspx.vb” Inherits=”WebApplication2.WebForm1?%>

2. You can enable the debugging in the application level for this
Edit the following trace value in web.config file

Enable trace enabled=true.

For Debugging the application first u have to put the break point into your application and after that Press F5 Key for run the application in debugging mode after that press the F10 key for analysing the error. It's give you the idea that from where erroe is genarating.

Second method od debugging is Immediate Window.It is very usefull when you want to debug your application