To add free shipping during checkout, you can choose between 2 options.
1. To set a general free shipping option that applies to all products in all categories, you can use this quicker option. Go to Configuration > Shipping Plugins, scroll down to the option called "Free Shipping" and click on the edit button. You will now see a screen called "Edit Free Shipping".
Complete the following fields
Minimum Order Total
Enter the subtotal money amount the customer must reach before free shipping is applied to their order. Only enter a number without a symbol.
Enter the zone that the free shipping should be available for.
Set this to enabled.
Set a sort order of priority for your shipping module.
Click on the save icon on the top right hand side of the screen. Free shipping should now be enabled in the front of your store.
2. You can use Custom Shipping in Shipping Plugins and get a lot more flexibility and options. For example, you can set it so that when a customer adds 5 quantities of a certain product to the shopping cart, he can get free shipping. However, to add a general free shipping option, in Custom Shipping, under Price Settings, set a 'Shipping Cost' of 0 and a 'Rate Type' of 'Flat'.
For more information about how to use Custom Shipping, see this article.