Sitemaps are a great way for getting search engines to crawl your sites and get every bit of data into their search engines. Obviously the main point behind search engine optimisation is to get your sites listed higher up, or more relevantly on search terms/keywords that you dictate, so letting search engines reach every last recess of your site is vital in achiving this. Currently the three big search engines are Google, Bing/MSN and Yahoo (although Yahoo may well be using search functions of other search engines very soon after its Microsoft takeover). So here are the steps you need to get your sitemaps listed:
Then go and find a script that will create your sitemaps for you. Joomla/Drupal/Oscommerce and many other open source packages have modules that will create sitemaps for you, so I would recommend using one of those, however, if you created everything bespoke/hand-coded (why?) then you can google for an appropriate script.
Then validate your sites and upload the sitemap:
The search engines will be along in a few days to crawl all of your site (hopefully), brilliant!