That depends on how exactly you would like to link your site to Facebook.
You can simply create a link to yoru Facebook page just as you would like to any other website, following the instructions in this video:
Or, you can install a plugin that creates an icon that is linked to your Facebook page. On this site, we use the Simple Social Icons plugin to create those social icons in the “Follow Us” section of our footer at the bottom of the site.
Or, you can use Facebook's own instructions to add a Facebook Like button to your site: https://developers.facebook.com/docs/plugins/like-button
It really depends on how and where you want the link to appear on your site.
Hope this helps!