Hi Ally,
First off, it is great that you are seeking some help on this. It can be very hard to be around alcohol if you have witnessed bad reactions to it.
If you do not want to drink (and for lots of reasons, you probably shouldn't if you are only 16)
Wellnowwhat's suggestion is a great one.
I would start the evening with "Actually, I am really just thirsty, could I have a water (or juice)." and then just never order a real drink. It seemed to make the evening be less about me and my choices and more about the reason why we got together in the first place.
That said, you will come across lots of situations where alcohol is involved. It's probably good just to be honest with yourself and others about how it affects you. In my experience being too paranoid doesn't help much either -- my husband's mother is a child of two alcoholics. Her experiences have scarred her so badly that she takes every opportunity to lecture others about drinking. Unfortunately that has led to her children feeling like they can't be honest with her about any kind of drinking or partying. I would even say that in their younger years they drank as a sort of rebellion. So while you can set boundaries for yourself it's good to remember that people can drink normally and that this is a personal decision that you cannot make for them.
Best of luck in working through this.