You need to use a div float like this: <div style="float: left; margin: 5px 5px 5px 5px;"> Your Ad Goes Here </div> The 5px is the margin size for the sides and top, bottom etc, you can change it as you prefer.
This is the easiest way to do it. There is a plugin out there related to adsense that does this for you and all you do is input something like <1> or <2>. I will try to dig it up...
If you go to single post, and right by the php function that fetches the content, put the code there. And if you want to mess around with it on your main pages and such... give this a look. http://www.tamba2.org.uk/wordpress/adsense/
yes, its adsense deluxe. and you cant put div tags in posts. The best way is to use adsense deluxe or editing the templates.
In case you need the link to the Adsense Deluxe Plugin, its here. http://www.acmetech.com/blog/adsense-deluxe/
If you want to put it in your homepage, open your main index template and add that code just where you want it, and if you want it in singlepages, open single.php