The best place to get information in on the specific search engines you want to place your advertising on.
Google offers two advertising programs: Google AdWords for advertisers and Google AdSense for Web publishers.
The AdWords program reaches people actively looking for information related to the product or services offered by your business, and sends targeted visitors directly to your Web Site. The AdSense program allows Web publishers to earn more revenue from their Web sites. AdSense delivers ads targeted to content pages, and when Google WebSearch is added to the site, AdSense also delivers targeted ads to search results pages.
Advertising on Yahoo! takes place through Overture, a wholly owned subsidiary of Yahoo!, Inc. Go to Overture.com for an overview of advertising services provided by Overture.