I had the same problem early on in my training. I used to think that not eating was the best plan. It really isn't! As others have said, it's very important to make sure that you have eaten (I found a couple of hours earlier was best and something quite light).
Plenty of water beforehand (and during if your instructor will allow).
All the air vents open full.
Try extra strong mints. Helped me feel less sick.
Don't let it worry you. Worrying about it made me feel worse. It is very likely to pass. Just relax and enjoy yourself.