You can easily add a free shipping rate to your site (e.g. for orders over a specific amount). Here's how:
1. Click 'Edit Store' -> 'Shipping,' -> 'Custom Shipping Rates' drop-down, then the '+Add Custom Shipping Rate' link.
2. Add the Name of your shipping (eg, 'Free Shipping - orders over $X.xx') and select the area you are shipping within.
3. Leave each 'Cost per...' field at 0.0.
4. You're done! Click 'Save' to finish.
If you have multiple shipping rates and one of them is free, you will need to instruct your customers (either on your website or in the 'Shipping Restrictions' message) on when they may select the free rate.