How to React to an Introduction Email
I find it worrying to see so many questions like this. They make academia seem like a terrifying place. It really isn't. Standard rules of engagement  apply (pun intended).


  So, someone (A) introduced me to someone else (B) via email, so that I can ask B questions and get advice. The email is directed at B, and I am copied. Should I wait for B to email me, or should I email B first? 


Unless cultural rules say otherwise, you should email B first. Beyond saving B time (in emailing you unnecessarily), you can ask for what you want.


  If the latter, should I reply to the original email and copy A, or should I just open another thread with B?


Reply to the original email, do not copy A, perhaps blind copy A, maybe mentioning you've done so to avoid bombarding them with further mail, e.g., Thanks for the kind introduction A (bcc to avoid further inclusion).

(Don't start a new thread with B. That thread wouldn't include the introduction and B may forget who you are.)