Controlling DC brushed motor for dummies, please
You first have to decide what signal exactly the autopilot is producing.  Is it something that says go more right or go more left, or is it saying go to this rudder position?  Note that one is the derivative of the other.

In either case, it sounds like you need a real control system, not just something that applies power to the motor sometimes.  You should probably implement a rudder position controller, then either use the signal from the autopilot directly or integrate it, depending on what it actually is.

The position controller will apply whatever current to the motor to maintain the required position.  If it doesn't take much torque, then little current will be applied.  If the rudder is at one end of travel and there is a lot of back force on it, then the system will automatically apply more torque.  The point is, what you really care about ultimately is rudder position.  So control that, then use whatever signal you get to adjust the desired rudder position.

On a separate topic, someone who "knows very little about electronics" shouldn't be messing with a flight control system.  You have no business being in there.  If you want to kill youself, that's your business.  But it's my business if you want to do it in a airplane because I might be underneath when it comes crashing down.