Some good info here so far. Like others have said the #1 thing is scouting. If you aren't where birds are you aren't going to get much action. It sounds obvious but most do not do it enough. In most areas of the state you will need a boat to scout. Most places you can drive right up to and check out will not be very productive.
The next thing I would add is hunt with people who are experienced as much as possible. Hunting with someone for one morning or weekend who has been doing it for years will give you more info than you could learn in a whole season on your own.....assuming they are ambitious hunters who know what they're doing. Don't be afraid to ask questions.
The last thing I would recommend to a new duck hunter is a duck call. If you are where the ducks want to be you do not need to call at all. You can kill plenty of ducks without a call. If you aren't really good at calling and reading the ducks you are going to hurt yourself more than you are going to help. You do need to know how to call geese but I would buy a duck call after you have all the other more important gear (waders, decoys, boats, motors etc).
1. Scouting & Location
2. Concealment
3. Spread (including motion)
4. Calling