Does your plane "twist" while still on the ground? If so, then you need to apply rudder to correct. Are you familiar with the terms "roll", "yaw" and "pitch"? When taxiing on the ground, your model can only "yaw" left or right. Yaw is when the nose of the model turns left or right. Pitch is when the nose points up or down. Roll is when the nose stays put but when one wingtip goes up and the other goes down. This is also called banking. Roll, or banking, you correct with the ailerons.

If your model is turning (yawing) suddenly after taking off, then your nosegear or tailwheel is misaligned.