Working with an organized group is a great way to learn, as well as books. Also, Google "duck rescue" and find a group in the areas you have access to. Usually, you are a volunteer and bear all the expenses, unless you eventually become such a valuable member that the group can subsidize some of the costs.
It's a great cause, I hope it works out for you. Be prepared for both heartbreak and joy.
I was just talking with someone who did animal rescue. She said she gets her ducks from the animal control and the dept of fish and game, apparently a lot of people dump (illegally) pet livestock into our lakes and the DOFAG has to go try and get them all out. It's horrible for the poor ducks, but at least you get to save some lives!
This is good info. I would love to start a duck rescue organization in my area, but I have no land to keep ducks and I'm not much for organizing fund raisers or running an organization. I mainly want to do the rescue and care part (I may be limited by allergies, though). Maybe, one day, I will find a good partner to help me.