Many young musicians have trouble with posture and the like while playing the saxophone. It is a difficult skill to master. I had trouble with it myself when I was beginning (I still do sometimes:). If you would like to know the most effective way to get a satisfactory sound from your saxophone... keep reading!
Think of the instrument as not the piece of metal you hold, but instead yourself. As Stan Getz, a famous saxophone player, once said, "If you like an instrument that sings, play the saxophone. At its best it's like the human voice."
To begin, the only way to produce a clear, accurate note is to follow it with substantial air support. Posture is the deciding factor in playing a strong note. Standing up while you play gives your lungs room to expand more than sitting would, and therefore is the most effective position. If you must sit, I would recommend sitting on the edge of a hard chair, never a sofa, with your feet flat on the floor and your back as straight as a two-by-four. If you sit like this, there should be no problem with producing sound.
Sometimes young music students will become lazy and slouch in their chairs. This can become a habit and lead to a lifetime of whiny music. The saxophone produces a very "whiny" sound when not enough air is applied. Also, it has a tendency to squeak. All of these issues are common with musicians, so do not worry if you are having trouble with producing a note. If you are interested in receiving further help, take a look at Sax Lessons.
photo is thanks to http://cincinnati.com/blogs/eyeontheeastside/2011/07/29/saxophone-player-takes-music-abroad/