I'm not sure if this is considered a sitemap, but I just enable my blog archives to appear on the sidebar at all times. I think it's a lot more convenient to have them there so guests won't have to click on menu buttons just to reach certain categories or articles, also it's a great way to let your guests preview each title of your blog so it probably betters you chances of a better bounce rate..
A sitemap is basically a "table of contents" of your site. Search engines like them. They are useful when the search engine crawls and index your site. If you want to set up a sitemap on your blogspot blog you can add a page using the "Page" gadget. Then you can do a sitemap manually, or get a program that can generate one for you automatically.
I don't know about Blogspot, but I know on Wordpress you can categorise your posts and set up a sidebar which displays categories, which is effectively a site map. Blogspot surely has a similar system? I'm sure there are ways to set up navigation sidebars based on most-used tags as well, but I haven't figured that out yet. Categories are much better for navigation, in my opinion!