Could I please ask you for your opinions for a complete beginner which of the free external software design programs is the easiest and most straightforward to learn - out of – Gimp - ink spscape - vectr or another. I want to use a free one at first to see if this is for me as I am very good at designing things.

Also our vector or transparent files absolutely necessary for T-shirt, mug , hoodie or other similar products  to put designs on?

Thanks Craig