Go to your admin panel and select Appearance -> Editor then find the template for your single page or post (whichever you're using). Create a new ad and copy that code. Look through your post/page template and find the part of the code that grabs the post/page title and past the code above that. If you want the code at the bottom then scroll down until you find something that looks similar to this:

<?php get_sidebar(); ?>
</div>
<?php get_footer(); ?>

That's the code to get the sidebar and footer. In most of the themes I've worked with, you can go up 2-3 lines (which are normally blank) and you'll find </div>. Hit enter to drop that div down one line and paste your Adsense ad code where that div used to be. (Div's are magical invisible boxes. You want the ad neatly below the page's content, you need to keep it in that div or create a new one and tell it where to go via the style.css file).

It might be easier to use a widget so you don't have to go in and edit code. There are several free wordpress plugins that allow you to paste your code in the widget and you can specify top middle or end of post. It will then go on all of your posts, and you can also specify pages. I've found that to be an easier approach then messing around in the code.

In your Wordpress admin just do a plugin search for adsense and you will find different ones. I like Adsense Redux.

I use that plugin too, much easier for technically challenged people like me to add adsense without messing up everything else!

Deprecated tag and doesn't always work.

Use:

<div style="text-align: center;">
ADSENSE CODE
</div>

You may need to add a few line breaks after this: <br /><br />