What kind of paint should I use to paint on T-shirts?

I really want to paint my own T-shirt designs on those regular 50% cotton 50% polyester T-shirts. What would be the most affordable but still effective paints to use? I heard that I could mix acrylics with a textile medium, or should I use go ahead and buy fabric paint? I would like to be able to paint bold, distinctive designs as well as softer, watercolor-y ones also. Tips on techniques, etc. would also be very helpful! Thanks in advance!