You are able to add a Like button which likes a given URL (static, e.g. your homepage) or/and put a dynamic Like button on your page to like the page you are actually visiting. The module add also a share button.
Install and enable the module.
Go to configuration -> User interface -> Facebook Like Button.
You can set the content types on which the button displays, choose to display it in the content block or in the links area.
There are several appearence settings.
Don't forget to give the anonymus user acces to the FB like button!
You can also configure Static Like Button Blocks in the Block Layout page. Go to block layout and click on Place Block in the region you want to use.