Answer to this question is much long, but let me explain briefly;
S-Curve is generally used for monitoring. It shows either we're ahead of our schedule or behind. But you asked How to work in it? So you have to go to Enterprise>Tracking OR you can access it via Projects window and click on Tracking Button. By default there might be two lines (curves) one that of BASELINE and other is that of ACTUAL. Now that can de displayed for Time as well as for resources. You can have all these options in the menu which pop ups when you right click over there.
NOTE: When ever you ask question, please clearly mention which user you are, that how much expert you are in primavera as this helps me to explain accordingly.
NOTE: Your question needs explanation with figures, Unfortunately, I can't do that here. Any how, good news is that I am writing book "HOW TO IN PRIMAVERA 6" which will be available on Civil Engineering Portal SOON. and of course, your question is discussed there in detail with figures.