Hiring an External Graphic Designer
Here is a list of some sites you can look at to hire a graphic designer to create your designs for you:
http://fiverr.com/ (5$ par design)
If you have decided to outsource you design to a designer, try to describe the concept as clearly as possible so that you get exactly what you asked for.
Visit these platforms and post your requests to applicants after asking each of the designers for a cover letter ensuring that there will be adequate and proper communication between you during the process. Also ask them for access to their portfolios to make sure that you have chosen the most competent one for your request. You may also need to include examples of T-shirts so that your graphic designer is aware of the quality that you want.
Remember that your design needs to be in a high definition format (.png, .jpeg, .jpg or .eps with a minimum of 300dpi). Therefore, your designer must know how to use Photoshop and Adobe Illustrator.
You can expect to pay a graphic designer up to 10€ an hour on these platforms.