1500m for your first tri? Flippen ‘eck, I take it you’re a good swimmer!
Ok, much of what charlie says but these also work for me (one ironman, lots of olympics and a few sprints):
Swim: go easy, pace yourself esp. over 1500m. I try to stay to edge of the pack if it’s an open water start cos I’d rather be pushed slightly to the side than pushed under by some over-agressive swimmer.
Swim-bike: put towel folded into a ft square under shoes. Sprinkle talc on towel so arrive at bike, put feet on towel and sort helmet out, then shoes on and off you go. If it’s hot don’t forget some sun block. Sounds ridiculous but I’ve raced in New Zealand where I fried because I didn’t spend 30 seconds slapping the stuff on my legs and neck (and they had sun block at every drinks station on the run and bike). Also, last two hundred metres or so change down a couple of gears and spin on approach to the bike/run transition so your legs aren’t pumped from pushing a 52-11.
Run: remember to take your helmet off. Socks are good (put talc in your socks before you start the race then shake the socks a bit), quck squirt of energy gel (make sure it’s open before you start the race) then off for an easy 10k canter (the first 2m or so are always a giggle).
Good luck!