The shirt's mesh should have a UV template associated with it which is normally found within the object file.  If such a template does not separately exist, you might need to rely on a third-party app to unwrap the object file and save the template as a .jpg file.  The .jpg image will need to be opened in another third-party graphics program so you may place and center the design of your choice.  It is best that you have a high-resolution image or text for it to be legible in the final render.