See if the forum below can help:
As with many forums you may have to register to go further than the page I found.
I have used a simple word processor (Microsoft Word is one) and I have used Adobe Illustrator. Illustrator is a bit more complex but makes placement of photos with text a bit easier. Then I use my own printer ( a Epson R220) to print on T-shirt transfers available at Staples (and, I am sure, other places as well). Then it is to the Ironing board with the T-shirt and transfer. It isn't hard.