Ok, first of all you need to set up your mouse.
Mouse Setup (PDF) or the help file describe how to do it.
Then, a plane has 3 major control surfaces:
Ailerons (ROLL the plane around its longitudinal axis)
Elevators (PITCH the nose up and down)
Rudder (YAW the nose left and right)
To fly, you generally only need the first two. For a turn, you roll to one side (bank) then pull back on elevator to start the turn. To stop turning, do the reverse.
When wheels are on the ground still, youll need the RUDDERS to steer (as you cant roll!). Use the A and D keys (S centers rudder) to taxi your plane.
Flaps are another wing surface that will add lift to your plane (ie help keep it in the air) at low speeds - You only need them for landing or very close and slow dogfights. You can also use them at takeoff, if you dont have much runway to gain speed or a very heavy plane (lots of fuel, bombs, rockets).
More help on getting started is in the Help File and on
Hammer's Net Aces. There you can also find a one page Quick Reference for the default keymap. Print that out and keep it close - it will spare you a lot of questions

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Welcome to AH devildog!