hearing audio while recording from an input

Recording in Mutitrack View, click on the little 'I" in the information box at the left end of the track(s) you're using.  This allows you to hear the input while you're recording.

One caveat though.  Listening to the input after it's done a round trip through your computer may result in unacceptable latency (delay).  What most people do is use an external audio interface that has "direct hardware monitoring as a facility.  This lets you monitor directly from the interface before the signal travels via the computer.