Most websites get into Yahoo by getting spidered from other sites. Meaning if you're already linked from another site or web directory, Yahoo's robots will eventually find you when they update their search index.
Go to DMOZ directory. This is the Open Directory Project begun by several key players from Netscape. Once your site has been accepted into the Open Directory, it may take anywhere from 2 weeks to several months for your site to be listed on Yahoo which also use the Open Directory data.
Another way to get listed in Yahoo is to use their own Add URL form located here https://siteexplorer.search.yahoo.com/submit