Before reading this you might want to have a look at How to verify your site with google webmaster tools, and How to find your google page creator sitemap.

Step 1) Go to the google webmaster tools website

Step 2) Click on the website that you want to add the sitemap to

Step 3) Click "Sitemaps" in the menu bar on the left of the page

Step 4) Click "Add a sitemap"

Step 5) Choose "Add General Web Sitemap"

Step 6) Type "sitemap.xml" in the text box

Step 7) Click the "Add General Web Sitemap" button

Your Done! The next time google indexes your website, they will find all your pages from your sitemap