That’s a very open ended question. How many different T-shirt styles? How many colors per style? How will you market to buyers? Will just have an online presence or will you have a local store?
I would research how other suppliers started - those who prospered and those who failed. What volumes did they start with? Was their success the result of a particular product or something else? Or, if they failed, what caused that failure? How does your local area factor into the equation? Do you know how to do things better locally because you know the area?