From your dashboard, navigate to Marketing -> Coupons, then click the "Add" button. You will then fill out the name, description of the coupon, and the coupon code you would to like to use. For example, if the coupon is for your Twitter followers, the coupon code could be "twitter". This is the code you will give to customers and what they will enter to get the coupon discount. You can fill out the rest of the form to configure the coupon to your liking. You can choose if it is a percentage or fixed amount discount (for example, $10 or 10% off), if a total amount needs to be purchased before the coupon will work (for example, $10 off a purchase of $50 or more), which products the coupon applies for, the dates the coupon is valid, and how many times the coupon can be used (both by individual customers and total uses by all customers). Then just click save and send the coupon code to the desired customers!
When customers check out, there will be a box for them to enter their coupon code into. Once they do this, the coupon amount will be subracted from their total.