How can we create a coupon code? Also, how do we keep tally on shirts orders?

Hello,

To add a coupon code you can go to the product list and create a coupon.

https://www.jotform.com/help/233-how-to-add-coupon-code-to-payment-forms/

What do you mean by keep tally on shirt orders? If you could explain what you're trying to do for this we can better assist you.