Site for you: Threadless.com
What it's about: Artists submit t-shirt designs, which will be voted and commented on by the Threadless community. If your design is popular enough, the company will use your designs for the next Threadless shirts.
How much you'll earn: If your design is picked, you'll get $2,000, a $500 threadless voucher (which you can redeem for $200 cash), and another $500 every time your t-shirt is printed.
What you need to do: Brainstorm your idea, download the Threadless submission kit that includes templates, and start designing your t-shirt.