Before your site will appear in search engine rankings they need to know that your website exists.
Once a search engine knows that your site exists, it will send a software robot to visit your site. The robot will log the content of your website, ready to present to users who are searching for relevant information.
If a known website links to your site some search engines will follow this link and find your site, however this will not work for all search engines and can take a long time to take effect.
Another method is to manually submit your site to search engines, each search engine will have a separate method of online submission such as:
Google: http://www.google.com/addurl
Yahoo: https://siteexplorer.search.yahoo.com/submit
MSN: http://search.live.com/docs/submit.aspx
AOL: http://www.dmoz.org/add.html
Occasionally, websites can drop out of search engines and directories. Regular resubmission ensures that your site is present in as many search engines as possible.
Some search engines and directories also require regular submission, in order to maintain a high position.