Hey! I've been doing some screen printing, and I've been using a 50% poly/50% cotton shirt which is working nicely-- it's pretty smooth, which is important so the ink prints clearly. I get my shirts from American Apparel.. they advertise which shirts are best for printing on.. you could use all cotton, but I prefer something mixed. Good luck & feel free to email me if you have other Q's (I'm not exactly a pro though!)
if you want to learn how to screen print. there are lots of good articles on the net. basic things are the screen and frame. squeegi
screen emulsion, ink.(all can be purchased at michales. in one box. speedball makes a compleet kit)
you can draw your ideas out then place them on transparency
use the photo emulsion to make the screen
print dry and wear.