Clip art: Double check you can use your clip art in products you plan to sell. Some allow, some don't.
American Apparel: This depends completely on your own preference, budget, and target retail price. Ordering samples is a great way to see with your own eyes what shirts you want to 'put your name on.'
If you mean getting heat transfers made as in plastisol heat transfers, here's a thread for you:
a review of 8 different custom plastisol transfer screen printers - samples followup
If you mean you make them at home, you need a printer like epson c88/c120, or any of your choice that uses pigment ink, and a paper like jetprosofstretch (tshirtsupplies.com), and a heat press. But it sounds like your designs lend themselves to plastisol or screen printing. Best of luck in the future, much success.
