I need to create a Sitemap for a Sitecore website. How can I do this?
It depends on what you need as a Sitemap. Do you want an XML sitemap that goes to Google? Or do you want a sitemap that shows the structure of your website?
I'd suggest looking at the Sitecore marketplace (http://marketplace.sitecore.net/SearchResults#query=sitemap), possibly downloading the source code to see how it's done. I think there's an example for both.
Otherwise, you can also create your own, but we'll need some more information - do you want to write it in XSLT or using codebehind such as C#?
If you're after a sitemap page to list the pages on your site you should try the Shared Source module 'Sitemap'. http://marketplace.sitecore.net/en/Modules/Sitemap.aspx
However, if you're after a sitemap for search engine optimization, use Sitemap XML. http://marketplace.sitecore.net/en/Modules/Sitemap_XML.aspx