Mallard ducks will fly away, so you have to clip wings. Have you considered a more domesticated breed, which will lay eggs for you? I just tried ducks out, and I have 3 breeds: 2 blue Swedish, 1 cayuga, and 2 Welsh harlequin. The Swedish and cayuga are 6 weeks old, and the harlequins are 4. The cayuga is getting a new home on Sunday because she's just too loud for my little neighborhood, but she'd do well on somewhere with more land. The Swedish are super quiet so far, but I think that's because they're both drakes... they're still young, though, so I'm not sure. The harlequins are my favorite so far... they were super cuddly as babies, and now they're just sweet and not too loud. And harlequins are supposed to be super egg-layers. But all of these are flightless, and won't need a really tall fence.
Consider where you live... do you have a lot of land, or neighbors that might get mad if they're too loud? Do you need it to produce a lot of eggs, to help convince your parents that it will be useful? Are you interested in preserving a rare breed?
My ducks live right along with my chickens, and eat the same food. I do need to give my ducks more greenery, so I'll buy a bag of frozen peas, or clip a bunch of lettuce or weeds from my garden. You don't have to have a pond, but they will love a kiddie pool. They do have to have a water source deep enough that they can submerge their heads, but a deep dog bowl will work if you're willing to keep it full. I have an 8'x8' pond, and my friend has a small water tank that she puts a heater in, in the winter.
I don't know what the laws are in your city. But here, if it's a duck that isn't considered wild, you just have to follow local poultry laws. In my city, they're all legal as long as the neighbors are fine with it.
