Hi Bernard, welcome to the T-Shirt Forums!
It depends on if you want to do the printing yourself (and what type of printing) or if you want to have someone else do the printing.
There are services like cafepress.com/zazzle.com that will allow you to upload your t-shirt designs and put them out for sale online with no up front costs.
Or you can read around the forums about the different printing methods and see which one you might want to get involved in. There are lots of posts that will explain what is needed to get started if you want to go the do-it-yourself route.