If you have received a coupon code by email, you can redeem it by clicking on Cart in the top-right corner of the screen, then click on the Checkout button. In Step No. 3, Payment Info, check the box for Have a Coupon or Gift Card?, copy and paste your coupon code into the Have a Coupon? field and click Apply. Your discount will be reflected in your total.
If you'd like to see a walk through video for this processes, see: How to Apply A Coupon (Video)