Hi,

As always, warm up your voice with simple vocal exercises, such as humming through ascending arpeggios etcetera before you sing. Go as high up as you can for your vocal range without straining your throat.

Identify the high note you want to hit. Ensure that this note is within an achievable range. Preferably, it was the last or penultimate note you could comfortably reach during the warm-up exercises.

Sing the note an octave below it first. Next, breathe deeply into your diaphragm, and simultaneously drop your inner jaw and raise your uvula (the little tongue at the back of your throat). If this sounds hard to execute in a split second, it is.

Place your voice high up in your head (above your cheekbones, or even above your eyebrows). It helps to imagine that the top of your head is big and empty. Now sing the high note with forceful thrust from the tummy area. You should have the sensation that you are hollering.

(from wikihow)