There are various ways to offer Free Shipping to your customers.
If you sell a product that should always get Free Shipping, select the "Free Shipping" checkbox in the Misc Settings on the Product form:
You also need at least one Shipping Method that is Free Shipping eligible, which will show up as an option for orders that contain all Free Shipping Items.
Sometimes, you want to offer Free Shipping that applies to all orders, or orders that exceed a certain dollar amount. For these cases, create a Manual Shipping Rate of $0.00 with the appropriate Conditions that need to be matched.
Other times, you only want to offer Free Shipping for certain Service Levels, like Fedex SmartPost. In this case, you would create a Real-Time Shipping Method, and select the appropriate Carrier Account and Service Level that you want to offer for free. Using this strategy, Free Shipping will only apply to orders that the selected Service Level applies to.
If you have a customer that you'd like to get free shipping on every order, edit their Customer profile, and check the "Free Shipping" box. If you want them to only get Free Shipping if they spend a certain dollar amount, you can enter a "Free Shipping Minimum", too.