can we make sketch on non planar surfaces .if yes then how ?
Momin has a good method.
Depending on what your goal is, you might also try:
Tools > Sketch Entities > Spline on Surface
This will let you sketch a spline which follows a surface.
But How to open a sketch on non planar surface?
The easy answer is you can't sketch on a non-planar face. But, as Momin and I pointed out, there are some options:
1- create a sketch on a plane, and project it onto the non-planar face
2- use tools - sketch tools - spline on surface
3- Start a 3D sketch, and use the spline on surface command
If you could post an example of why you need a sketch on a non-planar surface, I'm sure someone will have a good solution.