Are there any triathletes here?

Triathlon is a great sport. Makes you cross train and reduces the likelihood of injuries and they're a lot of fun (my longest distance, so far, is Olympic - there are quite a few member here who have done half and full iron distances).

For most triathletes swimming is the hardest sport. Before the race make sure you spend some time getting comfortable in open water. I learned that one the hard way. Swimming laps builds the fitness but when you do that first race and there's a mass start it can mess with your head big time (plus you want to practice sighting etc). Do you have a local triathlon club? If you do reach out to see if there are any informal swim meet ups (after my first race I was ready to chuck it in but another member invited me to their early morning swims and it made a huge difference in my comfort level). Practically every triathlete I know is generous with their tie and wisdom,

Practice transitions (there are lots of YouTube videos on laying out your gear on race day). Don't panic, taking a few extra seconds to get squared away is unlikely to impact your total time. Make sure that you have your bike helmet on before you unrack your bike and on until you rack it after the ride, depending on who organized the race taking it off early can result in a DQ. In terms of gear, if you have money to burn check out triathlon specific bike shoes (assuming that you have clipless pedals) they typically have a large loop at the back making it easier to pull them on and a built in soft line so you don't have to mess around trying to put socks on wet feet. Also it's worth buying triathlon specific shorts as they have lighter chamois that won't feel like your wearing a diaper after getting out of the water.

If your race is wetsuit legal consider renting one for your first race, I've only ever had the luxury of doing one race that provided strippers (help you take your wetsuit off not the other kind) so its a good idea spending some time practicing doffing your suit.

For the bike ride what you're comfortable on. If you've got a trusty old road bike use it. I'd probably been in the sport for 4 or 5 years when I broke down and bought a used TT bike, if you're so inclined you can spend a small fortune on a bike. If you look at the geometry of a TT they're designed to make you use your quads more during the ride, saving your hamstrings for the run. For your first race, comfort and familiarity are probably more important.

Practice fueling, I tend to fuel most on the bike and I swear by Gu Roctane. You can get 250 cal in a single bottle and because it's a drink it doesn't bother my stomach but try it before the race...nothing new race day.

Other than that.....have fun.