The only real "must" haves are a shotgun with non-toxic shot, a few decoys, and a call. Pretty soon you'll get snakebit and find out there are all kinds of things you "must" have. I "needed" a boat, a trailer, several dozen floaters, about 12 dozen full bodies, several layout blinds, a dog, a dog blind, a bunch of raffia and grass mats, heaters for the blind, a ton of clothes and waders, an ice eater, a mallard machine, a generator to run the ice eater, more shotguns, more calls, more calls, and more calls, a trailer to haul it all, and a four wheeler and four wheel drive truck to get it there.
The biggest thing you need is a good place to hunt and a way to hide. I had some of my best hunts over a couple dozen home made silhouettes and a blind made out of a section of hog fence. Now I have a small fortune invested, but I love every minute of it.