Softer reeds play low easier. Also depends on the tip opening of your mouthpiece. Smaller openings are easier to play lower. For me, plastic mouthpieces are just atrocious down there. Not sure what mouthpiece you are playing though. Also long tones are good for this, low notes require a bit of air, even at pp.

 

If the horn was shipped, the first thing I would do is take it to a tech to make sure nothing got knocked out of adjustment. If there are small, or not so small leaks, they can and will make the instrument far more diffucult to play.
You don't say what mouthpiece or brand/cut reeds you are using but most 'beginners' can start on a 2.5 with no real problems. If your lower register wants to jump an octave your embouchure could be too tight, or there is an mechanical adjustment issue. Hence getting the horn checked out before dropping reed strength.
As far as wanting to set your top teeth on the mouthpiece, it's perfectly acceptable.

 

Have you ever played sax before? How low are you talking about? If this is the first day you're playing, don't expect to be able to play a low Bb right away! Are you planning on taking lessons? Or are you just working out of a book on your own? or just playing with a fingering chart? You can rest your teeth on the top, just don't bite down hard with them.

 

I second Bandmommy's advice to get the sax checked. See Bruce Pearson Teaching the Saxophone Embouchure to review the basics. The key to playing the low register on the saxophone is to keep the back of the tongue down like singing "AHH", opening the throat (like the first part of a yawn), blowing lots of warm air, and keeping the embouchure firm (the same as the upper register). A good way to practice is start on a note you can play and then slur down to a low note and then hold it as a long tone.