i think before you ask how do you start a t-shirt business, you'll need to ask yourself..

how serious do you want to take your business?

are you trying to build a brand? do you plan to make a living doing this business? what's the business in 5 years down the road? are you even prepared to start a business?

once you've answered those questions yourself, you can then go about planning a t-shirt business properly.

- target market
- quality or quantity
- the right price
- the shipping
- the warranty
- the expansion

to design a t-shirt, you can either use Illustrator, Photoshop, Freehand, Fireworks and etc but it'll boil down to what software does the printer support. you'll need to learn about DPI as well.