I have a good advice for ya, bro. Don't even think about making these T-shirts beforehand. Make one, maybe two, show the result on the internet and then when someone would be interested, make 'em according to order. Otherwise you'll be left with lots of T-shirts that will sell nobody knows when.
It may sound obvious, but lately I met a woman who did exactly what I described not to do and now she lives with a giant box of unsold T-shirts.
Also, while on the topic, I was thinking about maybe trying to sell T-shirt with custom pictures. I don't mean pictures taken from internet, but rather pictures designed with original ideas from customers and picture done by me. I wonder if that would sell…