It depends on your reed quality and reed strength. For a beginning sax player, I would start at a 2 Rico. Never buy anything cheaper than a Rico reed, and increase your reed strength to half a size with a year or two. To play properly, fold your bottom lip over your teeth and place mouth piece in mouth. Your top teeth should be touching the top of the mouth piece, while only your bottom lip should touch the reed itself. Tip: when putting on the reed, only a sliver of black should be seen. You can adjust your tone by tightening or loosening your jaw. Don't just blow really hard. It takes air, but don't use an excessive amount that your cheeks puff out. That is not what you want. Also, adjust your neck strap to where you can play it comfortably. It is actually very important, since it holds the instrument itself. Tune your instrument correctly before even beginning to play. Just simply push the mouth piece in or out. It is basically the tuning. Go with the B flat scale on the B flat, or high G in alto sax. As you may have learned from your other instruments, intonation and sound need to go hand-in-hand. Air is also important in playing the instrument correctly. Make sure the pads and/or springs are in working or correct position. One spring unsprung will screw up the sound and alter it completely.
I would personally, from experience, start at the beginning since it will teach you the basics and what you need to know and learn. I have been playing alto sax since I was in sixth grade and I'm now a senior in high school. It does not hurt to start at the beginning. You will need to learn all the new notes, scales, etc. anyway, so it's best to start off there. Some helpful hints to know before I post this: when playing the b flat (the sax B flat), never use the keys that use the two index fingers. It is a complete waste of time to learn this key. Just use the concert fingering for B flat. It helps in the long run. Trust me. Also, clean your instrument. It helps keep it in tune. For a plastic mouth piece, soak in warm water to clean. For rubber, mouth wash. Change your reeds every one to two weeks and replace when chipped and/or broken.
I hope that you enjoy learning to play this instrument and I hope this has been helpful for you!