Roblox allows players to customize their look via wearing clothes that can be obtained through the Roblox catalog, which can be found here.

To start, check out these handy templates which ROBLOX themselves has created. If you right click and save these images, you can use them as a guide on how the clothes should look once you upload ...

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.

Each category has free in-game items for the avatar. Roblox hair, hats, heads, shirts, faces, neck, shoulders, front, back, waist, and a variety of additional accessories are all available for free.