First off is just using a simple "DIV" command. Generate your Adsense code for the Ad you want to display on your website. Place the Adsense code into your page, and at the top of that Adsense code place this HTML code: <div align="center"> Then at the bottom of your Adsense code place this closing HTML tag: </div>
<div align="center">
Your Adsense code here
</div>
If you use this method remember that you can also align your Adsense left or right as well. So it will look like this: <div align="right"> with your ending </div> I'm just using "center" above as an example. Please Note: If you use this method never forget the </div> tag at the bottom of your Adsense code. This closes the code and It needs this to work correctly.
The <div> HTML should work fine on your website, however some websites can have problems with <div> tags if the website is already using many of them. I have had problems with this myself in the past. Be warned: When using too many <div> tags, specially closing tags </div> it can really mess your web page up, so make sure you keep track of which div tags you have added manually that weren't already there. If you remember that you'll be fine, even if you get a problem. if your web page looks really messed up after adding div tags it's just a case of finding your <div> code and deleting it so your page goes back to the way it was.
The second method to use if <div> HTML doesn't work for you is <p> tags. So the same as above. Place <p style="text-align: center;"> above your Adsense code. And place: </p> below your Adsense code.
So it will look like this:
<p style="text-align: center;">
Your Adsense code here
</p>
Again, I have used center as an example and you can use which ever alignment you like.
If both of these methods fail then try this HTML above your Adsense code: <div style="text-align: center;"> and don't forget the closing </div> below! Have fun!