Silk screening is typically the easiest way to turn your own design into a T-shirt, but it's not cost-effective for one-off shirt designs. Over on Instructables, Ahixson shows you how to use bleach to make your own designs.
The process here requires that you make a stencil (which can also be used with textile paint to make a shirt), then apply the bleach to the T-shirt so it stains it. The end result is DIY shirt (or whatever other type of textile you have sitting around) that will last for a long time and looks pretty good to boot. Head over to Instructables for the full guide.
How to bleach shirts with custom designs [Instructables]