How does a pilot maintain stability for an unbalance loaded airplane? Consider Cessna 172 which has 4 seats, including the pilot, with the seat configuration as the picture below. Say the airplane has no passenger, only the pilot him/her self. I am not sure where it center gravity (CG). For longitudinal (roll), probably we can consider it separate between left and right seats. But for pitch, I don't have any idea.

My question is, if a pilot want to fly his/her airplane, how he/she assign stability control so the airplane will not has tendencies, either regarding the longitudinal (roll) or the pitch. What else control he/she needs to apply to make the airplane safely fly due to the unbalance load?
Use the available trim (elevator, and rudder if installed), and then manage the left/right fuel tanks if they are not feeding evenly.  Use the autopilot if so equipped, even if it's just a single axis wing leveler. 

With just 1 passenger, it is hardly out of balance tho. The POH has a Weight &amp; Balance chart as seen here  How are the limits of the center of gravity chart established?, plotting front seat weight and fuel will show the plane to be well within limits.