1. Don't buy anything until you figure out all your options. i.e. don't buy a really nice snorkel, you may never wear it. Don't buy the first BC you use, you may want a jacket style, or a back inflate, or you may want to go with a Back Plate & Wing option (commonly abbreviated BP/W).
2. Don't be afraid to ask questions, even if they are stupid, you will get some smart ***** comments, but you will also get the real answers, and the rest of us will harass the comment makers
3. Make sure that you find a really good instructor who has been teaching for a while. You probably don't want a brand new instructor, unless they have been diving for years themselves. Kind of just a policy of mine, if the teacher is young enough to be my kid, I don't want to learn from them, because they may have taken the fast track. A good instructor will not mind you asking them about their certification history.
4. When diving, and you kick something, STOP KICKING...sorry, a pet peeve of mine. If you kick something, you will float away from it, so there is no reason to keep kicking it...most likely you are killing the coral reef or just pissing off fellow divers by kicking them in the head.
5. Never be afraid to call a dive, or be afraid to not dive. Peer pressure will kill you in this sport.
6. Keep hydrated.
7. You can burp & vomit through your regulator, so if you have to, just do it
There is probably other stuff just as important, but these are the things I wish I would have known before I started.