adding a facebook 'like' button is very easy. facebook has made it very easy for you to just copy and paste a simple javascript code and then put it on your website. all you have to do is follow the instructions:

http://developers.facebook.com/docs/reference/plugins/like

The Like button lets a user share your content with friends on Facebook. When the user clicks the Like button on your site, a story appears in the user's friends' News Feed with a link back to your website.