like this http://www.bloggerplugins.org/2009/08/light-box-image-viewer-for-blogger.html under title and in post and how edit Readmore Button like same blog
deluxdon is right, you'll need to put the adsense code in the single.php file. Also consider adding it to the page.php as well. Also, don't forget that you can connect feedburner with it also and enable adsense to show adsense ads in your rss feed.
You need to render the code first so it can apply in blogpost template. Sorry I forget the way to render but you can search it trough Google. After rendered it, you can put it in blog post template depend in where do you want to place.