You can grab the info directly from the facebook developer page:
There's a 'get code' button there that will get you most of the way.
That depends on what method you've decided to use, but it does give you some instruction on that too.
So if we are following the instructions for HTML5 edit your theme.liquid and place for first chunk after the body tag. Add the second chunk to product.liquid -- wherever you need it to appear. I know those instructions could be on the vague side, but not sure how much, or little you know...