You need to excede the damage threshold of any plane or vehicle part for it to break. There is no partial damage yet, it is all or nothing until the next game revision (AH2), due in a little while.
Practice landing as much if not all of your fired bullets in a small spot on the part you want to damage. In every plane, there is potential for more or less ten kills (if not a few times that in some cases), if you hit the target's right weak points and waste almost no ammo.
You click-drag the gunset (the colored lines) you want to the convergence distance you want.
As for teamwork, just do whatever is needed most at the location you want to help. Don't forget to always have a backup plan to save yourself; sometimes the masses will do stupid things, don't forget to think for youself too.