As i learned flying my first cessna, prop planes love to pull to one side during a takeoff run. This tendency to shoot off to once side is due to several things, like prop wash on the tail, prop gyroscopic effect, and good old cross wind. When you take off, you have to apply rudder opposite to the direction you are veering off to. How to do this in the game? pick a spot outside of your plane, either in front of you or along the side of your plane (edge of runway works great), and use enough rudder to keep that spot from 'moving' as your roll down the runway. This is just like keeping your car in the lane when driving -- the road stays near a certain part of your vehicle as you look. If your joystick (you do have one??) does not have rudder action, you have to use the keyboard (sucks), so try to get a stick with built in rudder, or rudder pedals. Microsoft sells a good stick for around 40 bucks (sidewinder). Practice, practice, practice fying your plane slow and fast, taking off and landing. Your won't win many fights until you can handle your plane at all speeds.
good luck