The answer to that question kind of depends on what kind of artwork you want to create. Considering the fact most t-shirt designs are either pure graphics or a mixture of photo imagery and graphics I'd recommend getting familiar with Adobe Illustrator and then Adobe Photoshop. If you don't already know the difference between vector-based artwork and pixel-based artwork it's important to learn that early on in the process.
You can sell any original graphics or imagery you create yourself using Adobe software. But be careful about creating shirt designs that use branding/logos or other kinds of intellectual property from other companies. Unauthorized use can be pretty troublesome.