the body section of the code of a site probably starts in header.php and ends in footer.php of the used theme.
depending on the desired location of your ads, you migh need to add it to either of those, and/or index.php, single.php, page.php, sidebar.php, … ; or add the ads to a textwidget.
general:
http://codex.wordpress.org/User:Tomhanna/Adding_Ads_to_Wordpress
manu users are working with plugins:
http://wordpress.org/extend/plugins/search.php?q=adsense+&sort=
Follow this link it will exctcly guide you.
