Considering the fact that a decent screen-printing kit costs about $700, not including the price of the actual shirts and the inks and the time it'll take you to design shirts and print them, you'd probably be better off mowing lawns and washing cars.
Not to mention that you'd have to be good enough at designing logos and whatnot to actually find a niche in the already-overcrowded "rider-owned clothing company" market to be able to get your original investment back, much less make enough of a profit to actually be able to buy anything.