The folks at screenprinters.net will answer your questions and be really helpful and nice too. Post in the newbie section!
Providing your location for a question such as this could be helpful. Your process is multi fold: as you have a design, that part is out of the way. How many colors? You can find good blanks from American Apparel or from my favorite, Alternative Apparel, both are based here in the US and will add value to your shirt regardless of where you sell it, it's still Made in USA. The people at Alternative can give you directions as to where to go get your shirts printed because they are probably selling them blanks already. Good luck with your venture, it sounds like a business plan may be in order for you. Do a few inline searches and try to establish what your obstacles will be. Having shirts made may not be nearly as difficult as selling them.. good luck!!!