how do you make a transparent t-shirt on roblox?

It's the file, not the upload. Roblox keeps transparency fine if the png actually has an alpha channel. What usually happens is either the image was exported with a white background layer still switched on, or it's one of those fake "transparent" pngs off google where the grey checkerboard is literally part of the picture. Open it in something like GIMP or Photopea, make sure the background layer is deleted (you should see the checkerboard behind it), export as png and upload again.