1. What can I do to make it more enticing for ducks? I have seen a pair of mallards a few times a day but never more than 3 at a time. Only a very small percentage of waterfowl hunters do that. I know that what you read, watch, and are told differs, but look at it this way. Currently, the five -year average number of waterfowl hunters is only around one million across the USA. Of that one million, do you think the number that manage water or land numbers in the hundred-thousands, ten-thousands, is around one thousand, or rather numbers in the hundreds or even dozens? I am sure it is much fewer than one thousand and might be less than a hundred for all I know. Start thinking about waterfowl hunting differently. There are hunters (or guides) with managed habitat / water, fixed blinds, etc.. But the vast majority are mobile and that is what you should be doing at 16 years old especially.
2. Is it better to use a 12 or 20 gauge shotgun or is it personal preference? When I was your age, 20 gauge ammo was less expensive and just as readily available as 12 gauge. That is not true anymore. Particulary with non-lead ammo, the sub gauges including the 20 gauge, can be scarce on the store shelves and cost more than 12 gauge. Besides, 12 gauge gives you more punch - not as much as hunters will lead you to believe (because they don't test anything out, if they did, they would realize the difference is smaller than believed). So, I would suggest you go with a 12 gauge. I would also recommend staying away from semi-autoloaders and pump actions. Best is an over-under, second best is a side by side. Chose a model that does NOT eject shells or have a gunsmith modify your gun so it doesn't eject shells. I use a modified choke for ducks and an improved modified for Canada geese. Number 7 steel shot target loads for ducks and three (not three and a half) inch shells with number two steel shot for Canada geese, and stay away from high velocity ammo.
3. After you hit a duck, are you supposed to wait a certain amount of time before going to get it? Under the strictest interpretation of the law; you are required to immediately retrieve downed birds, make sure they are completely dead, and keep them with you and not mixed up with another hunter's birds.
4. After you retrieve the duck to you have to gut it right then or can you put it in an ice chest and wait a few hours? You do not need to gut birds or put them on ice for a long time. I am in the north, but even here as late as October, temps can and do get into the 70's (F). Birds can sit with guts in them, in fact, some hunters and some cultures leave birds intact for several days before gutting them. Some cultures cook birds with the guts still inside certain birds. That said, on rare occasions I have found that birds left all day when it is warm stink more when you clean them. The meat is fine, but the bird cleaning task is annoying, especially if the warm weather woke up the flies... Another law you need to be aware of relates to butchering birds in the field or before you get home > you must leave a wing and/or head attached to the bird. For some upland birds (pheasant) it might be legal in your area to just leave a foot. But if I were you, I would leave the head and one or both wings. Personally, I do NOT field dress any birds. I don't feel it is necessary, even when it is warm and buggy. I also do not want my dogs distracted then or the next time I return to the area. It also might fall under some littering violation as well. But whoever is going to cook your birds, should do some research - ducks and geese taste great, but they are slightly tricky to cook right. When they are cooked properly they are good, but when not cooked the right way they are not good. You seem to be thinking ahead, keep it up, but that includes the cooking aspect, especially with ducks and geese. If you use wax to remove feathers, don't catch anything on fire, read up on how to do it safely. I gut my birds first and then a butcher them with a method some people call "butterflying" and others refer to it as "spatchcocking". You should learn how to process your birds using that method, in my opinion. Small remnants of guts are easier to wash and pick out, you use almost all of the meat, and everything cooks evenly. Look it up. Good Luck and Happy Hunting!