I would recommend getting a vector software such as Illustrator, Corel Draw or Freehand, or look for a free vector program on the net, also you might get a pixel program such as Photoshop. You would then have to learn it and you might check
here for tutorials. Then, as you are just starting, I would suggest contacting a screenprinter. Depending on the design, you might also try vinyl transfers.
Start by outsourcing the job as you have no experience. If you see it really works, you might want to buy equipment LATER.
If you think you are not too creative or wont learn the design programs, then hire a freelance artist, it is of no use to buy a software if you are not going to make the most out of it.
Good luck!