C of G problem

Hi Can some one help please. How do you fix the C of G so that you are flying at the correct pitch. I tryed a flight tonight using the default 737 set up the flight through the flight planner,set the fuel to what it recommended,but during the flight the plane looked to be to heavy on the tail and at a guess it looked like i was flying just below the 10 deg mark How can i alter this. Thanks in advance Shaun.