Dear Rich Staff has a million of 'em!
Right, you had a question. First, you don't want to patent your t-shirt designs. Utility patents are for inventions and design patents are not intended for the appearance of t-shirts. You can read about the differences here and you can learn more about design patents, here. Second, you're best protected by copyright. If you don't want to spend much money, don't do anything. You get copyright automatically once you create an original design. If you want to register your most popular designs, you can do so online for $35 by following the instructions, here. Copyright registration provides benefits but is really only required if you are going to sue someone for infringement.