If you can, it's best to get at least 2 ducklings. They are very social and will benefit from having a buddy.
Caring for a duckling is very similar to caring for chicks. Feed them unmedicated chick starter, gamebird feed, or waterfowl feed. Supplement with brewer's yeast or crushed niacin tablets.
They don't need as much heat in the brooder as chicks do.
They are extremely messy and stinky, so you will be cleaning the brooder often.
They can begin swimming at a young age, some people let them swim for awhile at a few days old. I don't like to mess with drying them off, so I don't let them swim until fully feathered, around 4 weeks old.
Once they are feathered, they can go outside. Make sure their waterer is large enough for them to stick their entire head in. A pool or pond is not necessary, but they love to swim in kiddie pools.