From memory, these are the main characters (I'm being a tad stereotypical here, so the pink ones are girls and the blue ones are guys... hehe):
King Sextimus (mute)
Obviously, there are more males then females when it comes to leads. The *'s mean that the role is written for a male, but is sometimes played by a female. I do not reccomend doing that at all - it messes up the plot. Only make them females if you absolutely need to.
In terms of singing, the following roles need really good voices:
The rest can be spoken through most of the songs, save for The King who never speaks until the last scene (a curse was put on him).
For references, watching the 2005 Wonderful World of Disney movie will give you a great idea of the plot, but it's not a very good video. If you want to really learn what the stage show is like, I would suggest going to a library and borrowing (or ordering, because most libraries don't have it) a copy of the Broadway production, filmed in the 80's (I think).
If you have any more questions, feel free to PM me.