To see the curvature of the Earth, you must be at an altitude of around 35,000 ft or 10.5 km. This is about the cruising altitude of a passenger plane. However, to see it properly, you need a completely clear horizon (no clouds) and a field of view of at least 60 degrees. Both these conditions are rarely satisfied from the windows of your plane, but you can try.
Also, notice that the curvature is not visible even from the summit of Mt. Everest!