can anyone help me out here? I want to know which or all programs I can use to digitally design or draw a design for a shirt. I submited a shirt but it was blurry so the first one was a practice and definetly not my best work, but I would like to know what I can do to get awsome clear designs like a good threadless t-shirt should.

This was an awsome design and I loved it but the paper it was printed on feels life a paper you would pay 1$ for at office depot or something and the fact that I payed 30$ for the print I just expected it to feel and look a little more profesional like a real poster or something, but besides that I really liked it thank you.