From hand drawing to t-shirt graphic - help!

I have never created a design to be used on a T-shirt. Met a potential client recently who is an artist and he expressed an interest in drawing designs that would be scanned and converted to graphics for use on t-shirts. I gave him my business card and suggested when he is ready to give me a call.

Now, I need to know what the possibilities are and what kind of graphic I need to create that he can use. I suspect there is more than one method of creating printed t-shirts so what would be the most flexible type of file for me to create for him?

I am pretty sure he also has no idea of how the final t-shirt would be printed. Any information and suggestions would be greatly appreciated.