I have a new 12 wk old GR puppy and I want to use him for duck/goose/pheasant/grouse hunting eventually. When should I start training
him and how?

He is at the perfect age to start " traininging" him, but probably not what you envision as bird dog training. Now is the time for obedience, potty training, and all sorts of exercises geared toward making him a good dog. Aspects of field training need to be be gradually eased into his training regimine after he has mastered the basics of Good Dog 101.

Both Roebuck and Wolters have good information on the early months of dog training.

Good luck and keep us posted!