I'm Miles from the Social Care team at Shopify.
Depending on your theme, you should be able to add a memo (or notes) section on the cart page. Jump into your theme editor (Online Store > Themes > Customize) and change the dropdown to "Cart" and click "Cart page". From there, you should see a checkbox for "Enable cart notes". Once you've selected that, your customers will be able to add notes to their order if required.
As for showing the discount coupon section, this can be found automatically in your checkout. The discount code appears under the list of products and above the pricing. This can vary theme to theme, so if you let me know which theme you're using, I'll be able to confirm where it exists. Here's an example of where it appears on most themes:
One good thing to keep in mind is the discount field will only appear once a discount code is created. If you don't currently have any codes created, the field may not appear.
I hope everything's going well with getting ready to launch your store! Have you started marketing to customers yet? As you prepare to launch, it's definitely worth thinking about your marketing so you gain a loyal customer following. Think about using Facebook and Google Shopping, as they have enormous platforms full of potential customers for you. If you'd like to start developing a marketing plan, we have some great tools to help you along. Take a look at our blog and our marketing guides for more.
I also thought I would mention, we have a great design course through our free Shopify Academy, that I'd recommend checking out if you need any guidance.
Have a lovely day!