Aaaahhh... I'm torn between giving you a direct answer to your question and suggesting that you revisit this subject again in about 5 or 6 years.
Do you have a private teacher? If so, I don't want to contradict what he might say, but MY suggestion would be to build those chops and refine your breath control and technique for a few years BEFORE tackling the altissimo register.
But no harm in introducing the topic, and I'll save the cautions for last!
Altissimo is the uppermost register of the sax, above the high F you play with the palm keys all open. It uses the natural HARMONIC capabilities of the sax to produce OVERTONES higher than high F. It's believed that the saxophone was originally conceived as a 4-octave instrument, but over time, those highest notes became a bit of a lost art. But they are there, and the horn can produce those notes.
But it's not as easy, especially for a relative beginner, as pressing this key and that key and blowing. One must first master the technique of playing HARMONICS on the sax. When you play a bugle, you can play a series of notes without changing fingerings. This is accomplished by playing harmonics. You can do that on a woodwind instrument (sax, clarinet, flute, etc) too. For example, you can finger a low Bb, and without using the octave key or changing fingerings, you can play the Bb an octave above, an F above that, the Bb above THAT, the D above that, and so on. All without changing fingerings.
How do you change notes? By altering your air stream with your tongue and jaw position. AND -- get this -- by being able to imagine what the pitch will sound like before you play it. Sound really strange? Not really. Brass players do this all the time. They are used to it. Woodwind players, not so much. Takes time and practice. But you can't really master altissimo without that technique.
And before you tackle this sort of technique, it's best to build a good foundation of basic technique and breath control. That really takes several years for most students.
Yes, I know it's cool to be able to play those notes above high F, but I'd hate to see you trying to produce those tones by "biting" the mouthpiece or overblowing. Strong technique and breath control take time to develop, and I'm sure you will get there. Be patient.
However, altissimo fingering charts are everywhere on the internet, so I might was well tell you that high A is usually played with the octave key, and the middle and ring finger of the left hand. On most horns you might need to add the 1st and second -- and possibly third -- fingers of the right hand to make it play in tune.
Sorry if I sound like a wet blanket.
I remember being intrigued by altissimo too when I was a young player. Didn't get good at it until many years later.
Questions?
