Its really easy.
1. Get a google account if you already don't have one.
2. Login to your webmaster tool using your google login info
https://www.google.com/webmasters/to.../en/about.html
3. On the main page when you login into your google webmaster tool, you will see a "Add Site" box. Put the address of the site you are trying to add to google and say Ok.
4. Next go to
http://www.xml-sitemaps.com/ and put in the url of the site you're want the sitemap for (note this is the same site you added t o google webmaster tool in the above steps)
5. Once you create the sitemap, they will give you a xml sitemap file to download. Download that file and upload that to your web host server. (make you upload the sitemap xml file to the root folder of you site, i.e
www.yoursite.com/sitemap.xml)
6. Now go back to your google webmaster account and click on "add a site map"
7. Choose "Add a General Sitemap" from the drop down. It will bring a box asking for the link to your site map. Just put in the URL
www.yoursite.com/sitemap.xml and you're DONE
Once you're done adding your site and sitemap to your google webmaster tool account, google will take about 2 weeks to crawl your site. Once they have crawled your site you will able to see different statistics about your site. Google will also tell you if there are any broken link on your site, etc.
Hope this helps. If you having difficulty, just look around or try the google help files.