All a search engine spider knows of your site is the individual page it's currently crawling and indexing. To get it to crawl your other pages, you need to give it a trail to follow — and that's exactly what a sitemap can do.
It's a fantastic way to increase your chances of getting your entire site indexed as soon as possible, regardless of what page a spider lands on first!
The search engines require an XML sitemap (XML is a programming language). Find out how to build one here: www.xml-sitemaps.com. This is the easiest way to build one because you can simply enter the URL of your site and your XML sitemap will be created for you — for free!
(This XML sitemap is different from the one you put up for your human visitors. A simple index with links to your pages will help visitors easily navigate your site.)
A good "people" sitemap helps visitors navigate your site — and an XML sitemap does the same for the search engine spiders.