If you have a discount code and want to use it for your order, you will be able to add it to your order during checkout.
When you have added all your items to your cart and are ready for checkout, go to your Cart and on the right side you will see all products in the cart and you will have an option below that to enter a gift card or discount code. Once you apply the code you will be able to checkout using either one of the Express checkout methods or using your credit card for checkout.