If you have a coupon, you can apply that towards your order in your cart or on the billing address page.
From your cart, you will find a field to enter your coupon code beneath your total. On the billing address page, there is a link underneath the payment method fields that says, "Have a coupon or gift card?" Clicking on that will open the hidden coupon code and gift card fields. Enter your coupon code exactly as it appears, including any capital letters or dashes, and then press "Apply Coupon" to redeem.