This YouTuber put together an outstanding custom t-shirt video.  This video explains how to create a custom t-shirt using Adobe Illustrator.  She demonstrates how to create your custom t-shirt idea and prepare it for a screen printer.  One may find this method of creating a custom t-shirt challenging if they have never used Illustrator.  However, if you want to experiment Adobe does offer a 30 day free trial and you will find a pelthora of videos on how to use illustrator.  If you are not fluent in illustrator and do not want to experiment with this method of creating a customized t-shirt all is not lost!  TheShirtPrinter.com offers an online design lab that makes creating a custom t-shirt a snap!