Preprinted shirts with your designs are risky. I have been in business for over 11 years and have not seen one customer or shop succeed with this plan. Take your excitement for this industry and sell custom shirts with your clients designs or logos. Find an asi decorator in your area, and let them handle the artwork and printing. Not trying to burst your bubble, just stating the facts.
If you decide to print your own designs, copyright them first before you make your first sale. Google united states copyright office and follow their steps.
Either way, good luck on your new adventure