Everything, ready to fly, has to be in the airplane, Battery, receiver, servo's, ESC and motor. On a non electric plane it is balanced with no fuel in the tank but on an electric it is with everything. The battery is the heaviest element and is usually the part of the system that is moved forward/back to balance the airplane. The motor next heaviest but no choice where that is at. From front to back it is usually, motor, ESC, battery and servos/receiver. Airplane style dictates where the servo's and receiver are located. You don't say what kind of airplane it is so I can't give you a location for the CG. The CG varies any given airplane. Some balance at the main spar, some further back. Any instructions with the airplane? Usually shown on the plans or with the instructions.