Insert a "Shirt" into the same model as the humanoid. (You'll find it in Objects, or whatever that place is called.)
Then get the texture ID for the shirt you want, and paste it into the texture ID box in the "Shirt"'s properties.
~♫ Hobo ♫~
Copy and paste the ENTIRE texture?
Is the shirt you're using even a real shirt?
Oh, I also find it easier to be wearing the wanted shirt, and copy the texture from the shirt you're wearing.
~♫ Hobo ♫~
Then take AN texture AN make the TextureID one less than the ID. For instance, if AN texture was 34267, then you'd make it 34266.
Sorry, had to jump on the chance to make fun of your title :P