To download, install, and update Roblox on Windows PC, visit its official website and sign up. We have explained the detailed process in this article.

Steps for altering your avatar’s appearance in every Roblox experience, from adding and removing items to changing your skin ...

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.