If you are getting more rudder on one side than the other, you have to recenter the servo via moving the servo contol arm or making an adjustment at the clevis.

Here is how to do it at the servo:

Remove the Servo arm. turn on the radio and reciever. Make sure the stick is centered and the trims on the radio are centered. Now, re set the arm onto the sevo so that the rudder is centered. If you can't get it perfectly centered you will have to make an adjustment at the clevis.

Depending on kind of connection you have to the servo arm or the control horn on the tail, you can usuall screw one in or out do fine adjust the position of the tail.