The biggest thing I can tell you is if you wear contact lenses and really need them to be very conscious of your goggle choice. You *will* be kicked in the face when you swim- not once but numerous times. That will likely make your goggles come off or loose. If the swim is in deep water (where you can't just stand up and fix your goggles) you will be treading water while you fix your goggles and at the same time trying to keep water out of your eyes so you do not lose your contacts. That is VERY hard!! I ended up trying to swim with my eyes closed the first time so I didn't lose my contacts and it did not work because every few strokes I would find myself veering off course (I would raise my head and open my eyes to look). That just screwed me big time because the swimming is my strongest and favorite part and I found myself exhausted from all of the stopping/starting/getting kicked, fixing my goggles, etc. So be really confident in your eye gear and pack an extra pair of contacts with your bike so you can put them on when you get on your bike (that is if you can find it ; )
Another tip is be really clear where your bike is. If you are there early and it is not filled up you can easily lose track of where yours is in the big picture. You don't want to waste time looking for your bike.
Also, think about what your are going to wear well in advance. Make sure that what you are wearing on your bottom doesn't totally ruin your skin. You will be wet for a while and having wet against your bottom while you are swimming and then riding the bike and then running is a long time (and a lot of friction) on wet body parts. You can get pretty chafed up if you do not wear quick drying things (or things that are too tight).
Also, do not waste a lot of time and energy when you first get there walking around and looking at others, etc. Know ahead of time what you have to do when you get there (where you check in, etc.)and do not leave anything to chance. Also, don't worry about anyone other than yourself. "Run your own race" and just enjoy it. It is a very good feeling to finish- quite empowering. I think you will have a very good time!