Want to Add an Ad After First Post In Wordpress

I would like to add an add block after the first post in wordpress and I have no idea how to start. Do I modify my index.php file in my theme directory and if so what to?

Of course, the problem is that whenever I post a new topic, I would like the previous first post to appear past the ad, and the new topic to replace the last post.

 

Have you considered using a plugin ?

 

Is there one?

The only one I know about is Adsense Deluxe but this adds it to the post and I don't want that. I guess if there's a plug in that would allow me to add the add/ any HTMl after the first post, that would work!

 

I think AWSOM News Announcement plugin is what you need.
I use it with no problem.

It is available here:
http://wordpress.org/extend/plugins/awsom-news-announcement/