For the most part 99% of the artists on laFraise use Adobe Illustrator, which is easily the standard in the industry.
The following a are a few tutorials to help the beginner tshirt designer and even possibly teach the pros a trick or two.
Keep in mind these tutorials are fairly basic, and nothing beats getting in there and playing around, but they can help fine tune those skills.

Using the Pen Tool:

Using the Pathfinder Tool (Essential to know this tool):

Although not so much our style here at laFraise, this next tutorial can be fairly handy in other situations: