Well, to start with, you’ll want to fold your shirts so that the design is immediately visible. This is the first video I was able to find showing how to do so, but you can just do it by hand instead of using that dumb flippy thing.
What you do from there depends on how much space you want to take up. I’ve seen stacks of T-shirts set directly upon a table attract a surprising amount of attention. You can also pin them so that they stay folded, and then hang them up in the same way you’d hang prints! A couple of dollar store black picture frames can make this look pretty nice. And you certainly shouldn’t forget the canvas you don’t have to invest anything into at all: your body! Wearing one of the clothing products you have for sale is great for showing them off, particularly if you’re standing behind your booth rather than sitting.