If you have a discount code (coupon), you'll find a "Have a discount code?" field near the top of the main pricing page, before the plan/bundle selection section. Enter the code there before clicking "Subscribe Now" on any plan — the discount is applied automatically to that specific plan when payment completes.
Important points: every coupon applies its discount only once, on the first payment — it never repeats on any later automatic renewal. Each account can use a given coupon only once. Some coupons may be restricted to specific products or a specific billing cycle (monthly or yearly only) — if you see a message saying the coupon doesn't apply to your selection, try changing the plan or cycle, or check with whoever gave you the code.
If the coupon has expired or its usage limit has been reached, a clear message will appear when you try to pay.