The alto saxophone is smaller than the tenor and larger than the soprano sax. It is the most common saxophone found in classical orchestras and concert bands. The alto does not go as low as a tenor or baritone, and beginners often experience difficulty in getting the low notes to play on an alto. Although your first reaction might be to loosen the lips (or "embouchure") to hit those low notes, you should instead practice playing with a correct embouchure for all notes, which will enable you to hit the low ones and stay on pitch as well.
Play a middle A --- an 880 hertz tone --- on a piano, keyboard or other tuned instrument. This note is found six steps above the middle C on a piano.
Take your mouthpiece off the alto saxophone and blow into it as if you were playing. Try to match the piano or keyboard A. When you have correct embouchure, you will match the tone perfectly. Repeat this exercise for five minutes each day before you start to play.
Close your eyes and play a steady A note (now with the mouthpiece back on the instrument, finger your normal A, no octave key). Have a friend press the octave key when you are not suspecting it, and sometime later release the octave key when you are not suspecting it. The idea is that you should smoothly switch to the high A and back down without changing your embouchure. If the high A is flat or does not play, you need to tighten your lips a little when playing. If you do not go back down an octave when the octave key is depressed, you need to loosen your lips slightly.
Pick a starting note that is several notes higher than the lowest note you are able to play. From here go up the scale about three notes, then slowly back down, but end one note lower than the note you started on. Keep repeating this until you can play the lowest note. This exercised should be a part of your regular practice until you are able to hit the low notes with ease.
Inspect the pads on the low notes if you have tried all of the preceding practices for some time and are still unable to hit the low notes. Even a new or well-maintained instrument can have a problem with one of the pads. Take the alto sax to a music store or instrument repair shop and ask them to check the pads, especially on the lower notes. Pay for replacement if necessary. If you are renting the instrument, the shop will likely do this for free.