Useful stuff that no one ever tells you.
I have cycled for as long as I can remember, and take my word for it, that's a very long time. 🙈
There is the obvious stuff you need like a bike!! and you wont be able to compete in an event without a helmet, which you have to put on before touching your bike. But there is other stuff that's helpful, again that no one actually mentions.
Socks are an example of this, when you get out of the pool or lake, your clothes will obviously be wet, I found that invariably over the course of my bike ride, my socks ended up wet as well, simply because water drips off your clothes and down your legs. A fact of life!
I now never wear socks on the bike during a triathlon, I bought some really comfortable cycle shoes, or trainers if that's what you use, and cycle without them. If you prefer to have socks on, take an extra pair of socks, so that when you run you have a clean dry pair to put on.
Not good going off for your run with wet feet, its more than likely you will end up with blisters.
Another obvious one is check out the bike course, they usually have at least 1 hill in them and if you have to do loops - Blenheim springs to mind - you have that same hill to tackle on each loop. At Blenheim you have the same hill 3 times on the sprint distance, a lot of people get off and walk up the hill on the 3rd loop. Ensure your training includes the terrain type you will be cycling.
I always check that I can change an inner tube on my bike if I get a puncture, it usually isn't too difficult to get the tyre off, remove and replace the damaged tube, but oh my word, putting the tyre back on can be a difficult task, and, if there is no one around to help, it could mean the end of your race. Personally I now only use tubeless tyres, but not everyone wants to, or is able to make that choice.