Ditto Jacquard, and all of Penlowe's suggestions are great. I make my own t-shirts all the time, I have...more than 12 now. I only use Jacquard because it's opaque and high-quality, especially doing white on any color. Cricuts are nice but you do what you gotta do to get a stencil - I used thin tagboard before we got our cricut.
Fabric paint is pretty much a no-brainer. Heat-set it like it says on the jar once it's dry, and you just wash and wear like any t-shirt. Iron-on transfers on the other hand, definitely hand-wash.
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com