ThanksSimple software has the least tools for creating art. If you're just looking to do really basic art, then maybe something like Print Artist will work. Just make sure it at least is able to export to a supported format like ai, eps, or pdf. If not, then you'll be creating art that will be a problem come print time.
ThanksIf by simple you mean inexpensive, GIMP - The GNU Image Manipulation Program is free, as is Inkscape. Draw Freely. . I would recommend trying CorelDraw (they offer free trial) or Adobe (photoshop and Illustrator). Older full version of any of these can be had for under $100 and you would be much further along if the T-shirt thing takes off for you. Photoshop Elements would also work well of you are just doing pictures with some text and is an easier (and less expensive) version of photoshop.