If you want something simple, without needing an ad management plugin, do this:
1. If you want the ad in your sidebar, go to Appearance > Widgets and set up a Text Widget with the appropriate code and save.
2. If you want the ad in spots OTHER than your sidebar:
Open up the appropriate template file (e.g. page.php, single.php, etc...) and in the spot you want the banner to be, use code like this:
For page.php:
Code:
<?php if ( is_page(PAGE ID# GOES HERE) ) {?>
<a href="SPONSOR URL" target="_blank"><img src="PATH TO IMAGE" border="0"></a>
<?php } ?>
For category.php:
Code:
<?php if ( is_category(CATEGORY ID# GOES HERE) ) {?>
<a href="SPONSOR URL" target="_blank"><img src="PATH TO IMAGE" border="0"></a>
<?php } ?>
For index.php (if you have Latest Posts as your home page):
Code:
<?php if ( is_home() ) {?>
<a href="SPONSOR URL" target="_blank"><img src="PATH TO IMAGE" border="0"></a>
<?php } ?>
If you're looking for a full on ad management plugin though, I recommend Simple Ads Manager. It's a freebie, fairly light weight, and very full featured.
Hope this helps!
Cyndi