Do you mean paid submission to Yahoo directory or are you asking what to do to be your site included to the Yahoo search? To be listed in Yahoo directory you need to fill in the submission form and pay some money (not sure how much, if I remember well it was $300 for one year). To be included in the normal search you need only build some links to your website and wait. Yahoo works in the same way like google. It spiders the links found in the internet and when it comes to your website it will index it.
Usually SE's such as Google and Yahoo will automatically pick up on your site once it goes online. You can always be sure to have a robots.txt file in your public_html directory and you can also create a sitemap which will help everything on your site get indexed.
If you want to submit your URL to Yahoo, use this link : http://siteexplorer.search.yahoo.com/submit - Dynashox -
same goes with other seach engines... no need to submit your site. You just need relevant backlinks..
You can submit it by clicking here or just hang in there for a little while and they will index it themselves. I submit my sitemaps to Google and Bing to speed up the process.