This first triathlon is fairly short and rather easy. The sign-up desk is located just south of Vespucci Beach, outside of the Airport area. Once your character signs up they will change and compete in the triathlon right away.
Swimming Portion:
This one starts off with a swim into the sea. Get in the water and get ready to take a slight right. You pretty much stick to the coast here and the swim isn't too long, so keep following the markers. You can even go underwater if you wish so you don't have to worry about cresting the waves. Don't worry about gaining place here: just maintain. It is entirely OK to be in last place by the end of your swim.
Cycle Portion:
Soon you will be in the bicycle portion. Run to the bikes (sprint if you wish) and get to the closest one to you, even if its the last one, then take off. The bike portion is where you want to start to pass some people (which you will just by sprinting to the bikes).
The first part of this bike race follows the beach's bike trail and offers no room for deviation. Stay on the bike trail and out of the sand for max speed. As you work your way north you will eventually turn right onto (an unbusy) street.
Once you are on the street look for ways to cut corners. Checkpoints still need to be hit of course but they are often after turns meaning you can take the turn tight and save yourself time. Don't be reckless though: staying on your bike is the most important thing. When you reach the last bicycling checkpoint you will automatically get off your bike.
Running Portion:
This portion is where your stamina training shines. You are significantly faster than all other racers when it comes to running and should be able to outpace them all. There's only a handful of checkpoints in the running portion and cutting corners to hit them is much easier than on a bicycle, so keep at it and dig into your health if needed to win the race.
After the race is won you'll get a notification saying the next race has now been unlocked. Time for round two!