On the Roblox website, there are several templates. One can use those PNG files, which have a transparent background, as inspiration while creating the design for the shirt.

Only members of the Roblox Builders Club can make shirts and pants but no membership is required to make a T-shirt.

Much like most things on Roblox, you will be making this yourself from scratch, so you'll need to design a t-shirt in an image and then upload it to Roblox.