If you're just recording the voice track, there's no reason to work in multi-track mode, and as far as reducing noise, it all depends on what kind of noise. If you're recording in a professional studio, there shouldn't be any noise, other than breath, essing, the occasional flub, and other things which need editing. If you're getting too much noise, you should rethink where your recording, and what you're recording with, in terms of mics, pop filters, and such.
As far as levels go, in Waveform Editor you can look at Amplitude Statistics. If you don't understand what the RMS levels are, you need to do more reading. Here's another good, one page look at Audition: