You get a business account from your bank.
A business visa card will allow you to make purchases when you don't have cash for the purchase. If you don't have a solid plan to pay it back, you could find yourself in a big debt real quick.
Search the forums for shipping to find all kinds of shipping options and information.
http://www.t-shirtforums.com/tags/shipping/
You charge taxes to customers that are in your state. Contact your local state board of equalization to find out how to do that.