This is Dallas from the Social Care team here at Shopify. I would be more than happy to help you out with this.
To get a 'Free Shipping' label for certain products you will want to get an app. There are some great ome great apps to look into are
These apps don't require any coding so all you will have to do is download it in the app store and then you will have it attached to your store.
I hope this helps! If you have any other questions please don't hesitate to let us know. We have support here to help 24/7.