I have a fractured hand, and it's very swollen. will i have to wait swelling to subside before i get a cast?

No: You can still get a cast, but the cast may have to be split in order to allow for further swelling. Also, once you get the cast and your swelling goes down, you will likely require a cast change to a more snug cast because the cast will get loose as your swelling goes down.

Splint first: With marked swelling you will probably be given a splint to use until the swelling subsides, at which time you will be changed to a cast--but a lot depend on the location of the fracture and whether you need an operation.