If you can find a copy of Storey's Guide to Raising Ducks, or a copy of The Ultimate Pet Duck Guidebook, that will help you get set up and take care of them.
Meanwhile, they'll need a brooder that can be kept at 90F if they are less than a week old, dropping 5 degrees more or less a week until they're about ten weeks old. If you've raised chicks you already know to protect them from drafts, other animals, feet, falling, and other dangers. Ducklings can get into all kinds of trouble, wedging themselves between things, eating things they ought not, and more.
Ducklings need water 24/7 until they're at least a couple of months old. This gets quite interesting, because they are not physically equipped to be neat. They splash. More than you may imagine right now. They don't mean any harm, they just love water and have bills that dribble.
So figure out how you are going to keep the brooder somewhat dry, because paradoxically, these wet splashy ducks can get very sick if they are in a damp environment all the time. Try a search of the forum for terms like duckling water, or brooder water. We have had some great discussions, and there are many good setups. I used a splash catcher.
Ducklings need to be able to dip their little heads all the way in some water deep enough for that. They need that water to be available either all the time, or at least several times a day. This is needed to keep their eyes and sinuses and ears clean, or they can get nasty infections.
It is okay for them to swim a bit, even at a few days old, if the water is warm (90F the first week, etc.), it's only about up to the tops of their legs, they are watched constantly and taken out when they seem tired or the water cools off, which for us was about five to ten minutes. They need to be returned to a dry, clean brooder, and if they're not preening right away, dry them with a dry washcloth.
They need crumbles to eat at first, I added a tiny sprinkling of chick grit to their food once a day and began giving them very finely chopped lettuce when they were a week old. Ducks need three times the niacin chicks do, so if you are using chick starter, add niacin to the ducklings' water or sprinkle brewer's yeast on their food.
They'll wet their food, so it must be replaced once a day once it's wet.
They grow like nothing else I have ever raised. It seemed that overnight, their legs would grow a half an inch. Then their bodies would elongate . . . amazing.
They need to have water with their food always. They could choke if they don't have water. If things get messy in the brooder, do not even think about removing food or water until they are a couple of months old (and then for only about 8 to 10 hours). Figure out some other way to manage. Hungry ducklings will gorge themselves then toss the food back up, and be quite anxious and not as healthy.
There is a list of duck and duckling treats as a sticky on the forum.
At a few weeks old, they may decide they are terrified of you. It is a stage many ducklings go through and requires patience and treats.
I used a chick waterer and feeder for my ducklings, with a head washer setup and baths at least every other day. Carrying fifteen ducklings to the tub was a blast!
If you give regular food to a duckling be sure you have given it some grit first.
I know I am missing some things - these are just the things that come to mind.
Oh, and they are so much fun! And can really become people-lovers if they are handled well.