I can not say how stoked I am to almost be competing in an Olympic Distance Triathlon and actually feel ready to compete. Looking back, I notice there was actually a lot of stuff I did in preparation for this tri, besides training, that would be really helpful for someone else to know prior to committing to an event like this. Here is a quick checklist that I came up with.
Gear: Running Shoes, Running Socks, tri-shorts, tri-jersey, swim goggles, tri-wetsuit (I'd suggest renting in advance and testing before the event), bicycle, cycling shoes, helmet, cycling/running sun glasses, comfortable undies, body glide (this is a deodorant like stick that you apply to areas that chafe, it also helps you get in and out of your wetsuit), and four water bottles.
Nutrition: Energy gels to sustain energy, salt capsules to stay hydrated, electrolyte tablets to put in your water to stay hydrated, energy bar(s) to eat on the ride to sustain energy, and lots of water. ** Make sure to start taking this stuff a few months in advance and try different brands to see what works best for you.
Where to stay: Make your camping reservations/hotel accommodations well in advance of the event. You always score better camping spots and hotels the sooner you reserve. This will also give you an idea of what food and meals you will be able to prepare pre and post race.
Travel: If you are driving a distance, make sure to accommodate for delays so that you are not scrambling at the event to get everything sorted out. If you are flying, make sure to figure out how you are going to get your bike to the event.
There's still a lot of details that I left out that are more specific to the person such as food to bring, tools, first aid, etc, but the above should be a huge help to anyone thinking about doing their first tri.