You know how you're going to print your shirts. You have the equipment yourself or you know a printer that will give you a good rate. What you need is a design that will sell.
The first thing you should do is research, shop for tee shirts. The kinds of shirts you find for sale retail represent the kinds of shirts that sell well over all. Retail has a tight profit margin, there's little room for risk.
Now generally there are classic categories of designs that almost always sell