Well, I may be speaking out of turn but there are several ways to reduce stress while diving.
1) Ask questions - if you're not comfortable with certain equipment or are unclear on some procedures, ask! Your divemaster will be happy to go over the dive site, conditions, equipment, etc..
2) Breathe - take a moment to relax. Deep breaths in through your nose and out through your mouth can be very calming.
3) I've found that it's good to have your equipment set up well in advance of the time to "drop into the water." That way you're not rushed and stressing (and hot!) trying to get all the straps and zippers and crapola on just right with people waiting on you.
4) Review what you're going to do in your mind before you do it. "Okay, i'm going to grab that with my left hand shuffle forward, giant stride off with my right foot first". Picture yourself accomplishing those tasks and doing them well. Sometimes it's uncertainty about "what do I do now?" that can cause you a moment of panic. It's also a good idea to ask youself "Okay, if bad situation A happens, I'll do solution B". It's better to have thought of what you're going to do ahead of time, then trying to wing it on the spot.
5) If you still feel uncomfortable about a dive after doing all that, you can always say "No." We've all been there, and the "follow the pack" mentality if you're diving out of your comfort zone can get you into trouble.
Anyway - those are just some tips I'd recommend - there are many more experienced people than me on here that no doubt have some better suggestions.
Hope that helps!
D.