Here are the steps I would consider:
-Get a college degree that is related to the fitness industry (for example Sports Science).
-Chose the type of clients you like to work with and then go after a certification that will help you training them. CPR/AED are also a requirement.
-Get a liability insurance. IDEA has a connection with a great one (https://www.ideafit.com/fitness-insurance).
-Start from a commercial gym as a trainer to gain the necessary experience needed to train a variety of clients. This will also expose you to a number of other aspects of the fitness industry and help you become a better and more complete trainer. You can also find a mentor to help you with your development as a trainer.
-From your above interests which you have posted in your question, I think looking into triathlon training/coaching might be a good fit for you. If you haven’t already done so, start participating in some races so you can gain first hand experience of what it takes to be a triathlete (if this is what you’ll decide to do). It’s not necessary but this approach can be very useful and it can be used in other types of training that you would like to instruct.
These are some of the tips/steps I would follow if I was a newcomer in the business. I’m sure others here would give you some excellent advise and tips as well. I hope this helps.