adding ads to inbetween first and 2nd post

I would like to add ads after the first post. I know enough about php where I could do it myself, but I would like to know if there was a short cut so I didn't have to spend all the time finding out where everything is at.

If there is no way to do this, if you could point me to the right file and line where the new thread is created, I am sure I can figure out a way to do it.

Thanks!

Kyle

Here you may check example of Google Adsense which uses the same approach.