Not only do you want to make sure your site is included in the popular search engines, but you want to make sure that it shows up near the top of the results when people are searching for topics that are related to your sites. If you create a site about model railroads, you want your site to appear as high as possible in the results for searches like "model railroad" and "model train."
Unfortunately, search engines don't publish instructions on how to make your site rank near the top of their index. Before Google, search engines ranked sites mostly on the basis of their content. The more prominently a term was placed on your website, the higher it would be ranked for that term. So if your model trains page had "model trains" in a page title, or in heading tags on the page, then that page would be more highly ranked for the terms "model train." That's why you sometimes see pages with lots and lots of words listed in the titleit's an attempt to improve search engine rankings for those words.
At one point, search engines enabled you to provide hints about your site by way of meta tags. There was a tag that enabled you to specify a description for the page, and another that enabled you to list keywords associated with each page. The content of the tags was invisible; it was only used to help search engines with indexing. Unfortunately, people who published websites immediately started abusing meta tags, putting in keywords that were not related to the page content, or putting in far too many keywords in order to try to gain a higher ranking. Now all of the popular search engines disregard meta tags entirely.
Search engines now consider not only what's on your site but also who links to it in determining the relevance of your pages, so the more sites that link to yours, the better your placement will be in search engine results. There are companies that sell search engine optimization services that attempt to exploit this method of improving search rankings by paying popular sites to link to your site, or even creating fake sites and filling them with links to their clients. You should avoid these types of services, and be very wary of search engine optimization services in general.
Rather than worrying too much about how to make your site more friendly to search engines, you should worry about writing good HTML and making your site friendly for users. As your site gains in popularity, your search engine ranking will come along. One thing that can help is writing good, descriptive titles, and making sure to use heading tags for headings rather than just using large fonts.