Set your shipping costs as desired at Settings > Shipping costs.
Create a new discount at Management > Discounts / Surcharges. Choose the discount Free shipping, add a condition Total order amount equal or greater and enter the amount.
Optionally, you can set free shipping per country. Add an extra discount condition Shipping country or region. The free shipping will be visible once your customer has chosen his or her country.
Don't forget to activate the module Discounts.