I would say the answer to questions number 1 and 2 would be to experiment and try different things untill you hear something you like. It's a pretty tough question without knowing more about the sound you are after, the type of vocalist, etc. Kinda' like asking "What color should I paint my living room?" without saying what color your carpeting or furniture is.
Start out with some reverbs and short, mild delays, but don't be afraid to experiment if the music dictates. Usually I don't hear a lot of effects on rap vocals, though, come to think of it. I do hear a lot of double-tracking of the vocals, though (record two takes of the same vocal, and then pan them slightly off of one another or hard-panned).
The answer to number 3 is something that whole books, expensive education programs, and years of apprenticeships are based around. I'm afraid it's way beyond a homerecording thread. But some basic ideas would be to pan the various instruments, voices, etc. within the stereo field. Also, you could do a search on stereo micing, which can help to this end. Stereo effects are good, also (reverbs, delays, etc.)
