You will need to organize your stuff in the transition zone (zones), go to the bathroom several times, do a warmup, go to the bathroom some more, check your bike over to make sure all is working and that you haven’t lost any air in your tires overnight, and then you will, of course, have to go to the bathroom, so I suggest arriving at least an hour before transition closes (which is usually around 15 minutes before the first wave is to start).
Most races start in waves based on your age or predicted swim speed, with the professional athletes starting first. Before the first wave and while the waves ahead of you begin, you will be permitted to ‘warmup’ in a designated part of the water away from the swim start. Take this opportunity to familiarize yourself with the water temperature and the visibility/clarity underwater.
There will be very, very long lie-ups to use the port-o-potties, so if you simply need to perform a number 1, put your wetsuit on, stand ankle-deep in the water and pee. Try to eat plenty of fibre the night before (maybe with some dried prunes) to ensure that you will be able to rid yourself of most of your number 2 before leaving home or the hotel. This way you won’t have to line up for over an hour in desperate need of a bowel movement on race morning.
You will be nervous as you crowd together with all of the people in your swim wave/age group. Do not worry about this as everyone else is feeling the same (even those who have done this many times before. There will be someone there who you will be able to talk to, there always is. As soon as your wave begins, you will feel the shock of the coolness of the water for about 30 seconds, then all will be forgotten.