You will want to have as good knife to cut the leather. For safety. I prefer something that cuts without pulling it toward myself. A general leather knife does well to have a rounded tip so you can rock it or push it away from you. I made my own from a saw blade and it has served me for many years but there are lots of choices out there. I have used it for most leather work since I was in my early 20's.
A round knife can be handy but they can cost quite a lot for a good one.
Tandy has one for 49.99, which is the least expensive one to get if you want to go that route. They're not an absolute necessity. I didn't have one until a couple years ago and still don't use it a whole lot. You can find good used ones on ebay for $20-30.
You'll also want a
rotary punch or a set of drive punches for making the holes in it. Those can be had at almost any hardware store.
If you want to do decorative work, the
Tandy Lucky 7 tool set is great to get you started. For patterns, Al Stohlman's "How To Make Belts" is good.
You'll learn as you make belts if there's anything else you want to add to your tools. Feel free to ask for suggestions here. There are some outstanding leather makers on this forum.