Aria here, I'm on the Social Care team at Shopify.
Before you can create a t-shirt design, you will need to decide which supplier you are going to use and install their app on your Shopify store. You can find a list of suppliers in our app store, here. Popular options include Printful and Printify.
What sort of designs do you have in mind? Have you already created them yourself or are you looking to hire a designer?