Short Answer: all of it is important. The more you can retain, the better off you will be.
However, some content is definitely more important than others.
Vectors, kinematics, dynamics and electromagnetism are absolutely vital.
Waves and thermodynamics are less important. After Year 11, you probably won't ever be examined on beats, doppler effect, Snell's Law, thermal conduction, convection, latent heat of melting or specific heat capacity.