If you received a gift card or discount code and would like to apply it to your ticket(s), you may continue along the purchase process until you reach the payment screen. At that point, you can enter your code in the box which says "Gift card or discount code."
If you are using a gift card, once you hit Apply your code will cover the base price and any applicable fees up to the amount available on the gift card.
If you are using a discount code, once you hit Apply your code will subtract the proper amount from the base price of your ticket(s). You may verify the amount by looking at the ticket price and checking if it was applied.