Facebook “Like” buttons are generated and styled by Facebook, so they are consistently easy to recognize - usually small blue buttons with the “f” icon on them, and sometimes with a Like “count” next to them, e.g.:
A Facebook “Like” button is not an element that is built into your theme. Facebook has instructions on generating the required code here: https://developers.facebook.com/docs/plugins/like-button
However, this is considered a third-party integration. If you do need assistance with adding a Facebook "Like" button to your shop, we have recommended Shopify Experts and custom developers who can help you implement this on your shop!