What medium may I use to design for a Tshirt screenprint?

I am designing a Tshirt for our small club, and we worked out a design. I'm pretty good at art (OK, no training, but I'm still not bad) but I don't know how to get my idea on paper. I'm wondering if I can do it in oil pastels so it has a sketchy feel? Or do I have to ink it with markers or something? Or nowadays, is it best that I just do it on the computer (which wouldn't be as fresh, at least for the way I work). Any insights would be great!! This design is going to get silkscreened on, I believe, but I am not the Tshirt ordering person.