Hi Guys,
I hope this comes across OK.
Assuming your set up is OK (Sax in good order, reed and mouthpiece manageable and in good order).
There are many things that can effect producing a note that you will one day get right and just take it for granted.
How's your embouchure?
Not too tight, your jaw/lips will soon ache.
Not too loose, you'll leak air and probably slobber the sides of the mouthpiece.
Bottom lip should be over your bottom teeth and allow the reed to rest gently on it.
Top teeth should not bite the mouthpiece, though this seems to be less important as many good players have gouged the beak of the mouthpiece over time.
All in all as a beginner you will tire quickly but each time you pick up the sax, think comfortable embouchure.
Breathing
There are many good explanations of correct breathing and air support and Pete explains it on the main site.
In short, you need support/control from your diaphragm to release a full body of air consistently.
Posture
Make sure you can achieve the two points above by being in a comfortable standing position. You can sit down but to start with, as you absorb the benefit of good embouchure and breathing, it seems easier to think about and monitor from a standing position in my opinion.
They are the basics.
When you learned to ride a bike, what stopped you wobbling about and finding good balance is probably hard to describe...but you did it right? so have confidence, its a matter of time.