Experiment in paper. Try diferent measurements, play around with type and graphics... Remember that is much more cheaper/easier to do if you use only one color. Once you're happy, get them silkscreened, or try it yourself.
I've been taking clases, and as my intention is to print limited editions of t-shirts, it is far more convenient for me to make them myself. If you have them done you may bump into large minimum amounts.