Nick here, A Shopify Guru.
We have a great doc online about creating and using online discounts. I'd be sure to check it out :)
When you are viewing your admin, your URL is the-jewel-fix.myshopify.com/admin.....etc. The /admin part indicates you are in the backend, or admin part, of your shopify store.
Your public facing site is the-jewel-fix.myshopify.com. If you go to that address, enter your password, you'll be presented with the front-end, public facing (Once live) part of the site. Add something to the cart, and progress to checkout. On all the checkout pages, you'll see a field to enter discount codes, as in the image below:
The codes are entered into that field, and applied once the button is hit. Your checkout is for the moment disabled until you choose a plan :) You can use our doc on testing the checkout to see the field for discount codes in the meantime.
Hope this helps!