For landing, put
plane1 land "land"
in a waypoint or trigger and your plane will head for the nearest operational airfield and, you've guessed it, land. Look at the [un]official command refs for more info.
Take off is automatic from an operational airfield I think. There is only one operational airfield per island. If you want your plane to fly at a particular height use the flyinheight command. If you want a plane to start a mission flying make sure its special box (where you can have In formation) is set to flying. If you want a small moment of amusement place an A10 on the ground (somewhere not on an airfield) with a distant waypoint and watch him try to taxi there.
Have a wee search of this forum, I seem to remember this has come up before.
Hope that helps, let us know if you have any specific problems and we'll try and help.