Ok, first a few clarifications on how you actually "steer" your plane while flying (or while taxiing on the ground). You use its control surfaces. Moving them changes the airflow and thus the direction your plane is going at - much like turning the steering wheel on your car moves your tires on the tarmac which in turn makes your car change direction.
In this picture you can see the Surfaces used to control a plane:
To change direction of travel with a plane, all surfaces have to "work together". Each of them changes one axis of movement.
Ailerons: they make your plane roll around its longitudinal axis.
To control the ailerons you have to move your Joystick/Mouse left and right.
Elevator: Finally the answer to your question . The elevator makes your planes nose go low or high (ie pitch).
To control the elevator, move Joystick/Mouse forward to nose down (dive) and towards you to climb.
Rudder: The rudder makes your nose go left and right. While in the air, it is not of much use to turn, it mostly causes your plane to fly "sideways" and helps roll You need it later for advanced maneuvering and fighting. It is very important to steer the plane while on the ground though (taxiing on the runway) and for takeoff (to keep plane straight).
If you have a twisty Joystick, you can map rudder control to the twist axis of the stick. If you do not have that or are using a mouse, use the key commands for rudder: A for left, D for right and S to center rudder.
Flaps: At low speeds, they help your plane keep its nose up as they create additional lift (ie pull the wings upwards). They are used for landing and when in real slow and close fights.
Now for example, to make a turn while in the air (i said rudder isnt really effective) you need ailerons and elevator working together. Roll your plane to the side you want to turn to (about 45 degrees) and then pull back on elevator to start your turn. To stop, remove the elevator input and roll back upright.
More basic maneuvers can be found on the Trainers Homepage.
Now back to your stick setup in Map Controllers:
The respective axis on your Joystick simply gets mapped to the analog input in Aces High... you can reverse axis if you prefer that for some.
X Axis - Analog input Roll (Aileron) - move stick left/right
Y Axis - Analog input Pitch (Elevator) - move stick forward/backward
Z Axis - Analog input Throttle1 - move throttle lever
Z Rotation - Analog input Yaw (Rudder) - move twisty/rocker
The other axis/rotaries are the sliders and rotary buttons (if you have any on your stick).... if you want to, you can map those to Trim functions or any other analog input.
If youre in doubt about wich axis is which - move it on your Joystick: The numbers in the respective brackets (on the list) will change.