Answer:
You can print up special printer papaer that allows you to iron the design you print out directly on to your t-shirt. However, the problem with these is that they don't last that long before the design starts to come off in the wash.
So if it is a t-shirt you want for repeated use I would look to go to a specialist store, if you don't know of any ask a local sports team where they get their t-shirts printed or try a reputable online dealer.
you can go to a specialty store...upload your design..they print it...i did one shirt at teedude.com...you can do more, but i just needed one for a special occasion