Welcome to waterfowl hunting. If, as you say, you are new to shotguns and waterfowling I'd be inclined to do the following.
I'd look for a good used 12 Ga. shotgun with a 28" barrel with either a fixed modified choke or choke tubes. A 3" chamber will more than suffice your needs. I'd also start with a pump shotgun. There is alot of excitement hunting waterfowl. Untill you've experienced all the adrenalin rush, a semiauto will just keep firing after each trigger pull. Not always a good thing when one is really excited, especially when hunting with a group. Best to start out, in my opinion, with something less automatic. As time goes on, I'm sure others you hunt with will let you try out there shotguns. After gaining alot of experience and confidence, you will have better knowledge of what you need and want out of a shotgun. Spent some time and ammo getting to know your choice of shotguns. Good luck to you.