start by first getting photo shop and illustrator and taking a few collage courses or how to use them. and one or two biz courses. I would start out with pb and or t-shirts. find one or two good local screen printers as see what they can and cant do. and there cost per run. ie set up fee and cost per shirt. a lot of t-shirts printed and non printed are made in china and shipped here. there a few web sites like http://www.alibaba.com/
that puts comp in contact with other comp. you also want to talk to a cpa as there things you can write off for biz start up and also talk to local small biz rep they can show you if there are grant money or loans for start up. the small biz rep will also tell you if you need to go to town hall or with the state and reg your company.inc or dba.
you also need software like peachtree accounting for the books. also a biz checking for the company.