Why stop at one t-shirt? Think of the possibilities!
There could be a summer t, a winter sweatshirt, a sleeveless tank top (hey, we're hot here in the summer in Texas), hats, and you could offer white, black, colors, etc.
Yes, you should start slow, with one design and one type of shirt. Just make sure it has a simple design which could be adapted to other products.
Also, remember copyright and trademark considerations (Mr. Reiner might sue for copyright infringement if you use the the 'Spinal Tap' font, for example).
Also, coffee mugs are a popular item...
I hope to see an orderform soon!