If you receive a discount code as a first time customer, you will need to enter it on the page where you choose a shipping method. The shipping page is visible after you click "Checkout" while viewing your shopping cart.
On the upper right-hand side of the page, you will see the form below that says “Gift card or discount code” and the button that says “Apply.”
Just enter your discount code in the box and click the “Apply” button. If you do not click the "Apply" button you will not receive your discount.