Storey's Guide to Raising Ducks, or The Ultimate Pet Duck Guidebook would give you loads of help!
Meanwhile, I will try to list the first things that come to mind.
If you have found duckling food, that is wonderful! If you are feeding chick starter, there may not be enough niacin for some ducklings (they need three times the niacin chicks do), so sprinkling brewer's yeast on their food, or dissolving a 100 mg niacin capsule in a gallon of water will help. Once they are outside eating bugs, it is less of a concern.
They need, need, to wash their little faces frequently to avoid infections. A daily bath or two is good, many people make a head washer they cannot get into (margarin tub with a hole cut in the lid, for example).
They splash water a lot (perhaps you have noticed?), so managing that will help keep them healthier.
Baths are fine at an early age in my experience, but when I say bath, I mean at first, 90 degree F water up to the tops of their legs only, close supervision, and they usually tucker out after about five or ten minutes at first. Then get them back into a clean warm dry brooder, and wipe off any that don't immediately start preening with a dry washcloth.
Keeping them at a comfortable temperature starting around 90F the first week and dropping 5 degrees a week is a rule of thumb, but actual practice is all over the map. I followed that rule of thumb with my runners and they did fine. The ducklings need to be able to get away from the heat source, as some may overheat at the usually recommended temperatures.
Ducklings will get into surprising trouble, eating things they ought not (sometimes bedding shavings or pellets, pieces of plants or plastic, etc.). One woman showed us a photo of a duckling inside the reservoir bottle of a waterer!
I also began putting just a sprinkling of chick grit on their food once a day (perhaps a teaspoon). After Week One, I began giving them very finely chopped treats and small peas. They will overeat, so go light on the treats at first.
They are susceptible to harm from feet, other pets, falls, drafts, moldy feed, chills, digestive impaction, and various infections. Still, they generally sail through ducklinghood without any of these problems.
After they are grown, you'll need to watch out for bumblefoot, which may be one of the most common ailments of otherwise healthy ducks.
I am sure I missed a bunch, but I will send this on with prayers for you all, for much fun ahead!