Welcome to a disease!
You have all the gun you need, it will shoot 3.5" if you ever need to though I never do. Many Goose shooters go that route though. You'll want to get into some decent Camo clothing. Concealing movement is extremely critical when waterfowl hunting. You'll obviously need to tailor your clothing needs to the weather and surroundings you will be shooting in. Out here in California we can shoot the opening few weeks in T-shirts but by mid and late season it's time for the 4 in 1 type coats if you're in the north.
Face mask! Don't leave home without it.
If you're in the marsh than a set of boot foot waders will probably be necessary. If you're hunting cut fields than perhaps a bib type overall will work. For your first time maybe utilize appropriate clothing that you have and just conceal yourself with a camo shell type garment until you can figure out what is best for you. Quality waterfowl clothing will be spendy so research you needs and buy the best and buy once. I can heartily recommend anything built by Drake Waterfowl. I am on my second season with their stuff and am buying more all the time. IMO, they are second to none in design and comfort. Duck Hunter's clothing built by Duck Hunters. If you are not comfy and warm than you will not like being out in the weather....weather is what the ducks/geese fly in so be prepared.
A guided hunt might be a good idea in your area for your first time as not only will you get into birds but you'll learn a lot doing so. Hopefully a guide would be able to instill some proper etiquette as well as proper technique. If you by a duck/goose call, leave it at home. Unless you are a child prodigy with musical instruments, you'll suck at calling for quite some time so practice while listening to some instructional Cd while driving around in your car. Most importantly, go to a refuge if you can and sit in a closed zone and listen to the orchestra of waterfowl. Be the closed zone.
If you have friends that are into it, pick their brains or go to a local gun shop and befriend a fellow hunter. I have had the opportunity to get several guys that I never knew out in the field for their first hunts. I've always done my damnedest to show them whats right and whats wrong wth the sport. Each and everyone is thoroughly addicted now.
Join Duck's Unlimited and Delta Waterfowl. If you want to continue hunting, their support is mandatory.
Post more questions here...there's always good support to rely on.
My name is TreeDoc...and I'm a Duck-a-holic.