I'd say to start with the basics. You probably have many of the skills already, making and following a pattern, cutting the material, dying, stitching and finishing.
Personally, I needed a holster so I got myself a scratch awl, sturdy scissors, some blind cord, a snap set and a $20 hunk of leather.
That was many years ago and now, my last restocking order was over $300, and I forgot to order some 1/4" Chicago screws.