How to add extra planes?

Try this:
-Open a sketch on a surface of your model OR in any other given plane
-Draw a simple line (but think where you start it and in which direction you point it)
-Exit the Sketch
-Go to: Insert > Reference Geometry > Plane...
-Select the line, and it starting OR end point
You will see the new plane will be perpendicular to the line, and the line will hit the plane on its start or end point (whatever you haev chosen) as if the line is the third axis (Z) that is not in the plane (X,Y).
Keep that orientation in mind when placing your line! I suggest you try it out first (planes never hurt anyone) to see what is going on, and than put the plane where you dearly need it.

This works on the ends of arc and splines as well