I had a store in New York and another in Portland. Your attention on lighting is smart. I'd suggest that first of all you take an afternoon and check out what other stores (similar operations to you own) have done. This will help with ideas about how to light and what sorts of lighting systems will work for you.
Your budget will be important as will the existing electrical system in your building. I presume it is up to code and you have a good circuit breaker but that's something you should establish.
You can then visit stores that carry lighting (Home Depot, Ikea). Specialty lighting stores tend to be very expensive and would likely be a waste in a T-shirt store. You really don't have to spend a lot.
If you go around to other retail operations like yours, you'll not only get ideas about lighting but about display as well. That's important. Last advice - be creative.