You can use an online tool or do it by hand (oldschool).
I highly recommend GSiteCrawler by SoftPlus.
Google Sitemap Generator for Windows :: GSiteCrawler
It's free to use, has a short learning curve and it'll create .xml sitemaps, robots.txt and Yahoo! Urllist.txt files. It'll even upload them to the server for you if that's what you want.
Make life easy for yourself, give it a shot.
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