How can I apply/put a t-shirt on a user when they click a TextButton?
Use the MouseButton1Down event of the button to detect the click, then just configure:
plr.Character.Shirt.ShirtTemplate = yourShirtIdSpeaking of T-shirts, they are similar to Shirt and Pants.
Here’s the API reference, you have to create an instance of this if the player doesn’t have it. Change ShirtGraphic.Graphic to any ID that is per button.