On the cart page, after you've selected some orders, you will be able to put in your gift card code through the section labeled "Gift Cards". You may either check the balance or apply the gift card to your order.
On the checkout page, after you've selected some orders, you will be able to put in your gift card code through the section labeled "Gift Cards" under the Payment Section. You may either check the balance or apply the gift card to your order.
Alternatively, if you've created an account, you can go into "My Account" and navigate to "Gift Cards" on the left navigation panel. You can redeem your gift card so it stays on your account without needing to find your code every time now.