Here is my particular road to triathlon. The road will be different for all, but there are common themes worth reciting:
First step I guess is to get active, get into some sort of sport. I got that from my upbringing, I was always involved in one sport or another, never too deeply, and at times very superficially.
Second step may be to become interest or downright passionate about a particular sport, doing it frequently, trying to get better at it. This was biking for me, which I started doing as a commute around 2003, and fell in love with around 2005, when it became more than a form of transportation.
The next step for me was to start measure athletic performance, like tracking speed, distance, or some other form of endurance. Knowing what your best is, and trying to beat it when you can... In my case, I started to pay attention to my average speed on a certain route.
I then realized that one sport is not going to cut it, and added in another. I started running, to improve my cycling. I am not sure it made me a better cyclist, but I certainly fell in love with running.
The fifth step was to develop interest in participating in events, and racing. The first events will likely not be triathlon races, but more likely a run, or a bike event, maybe a duathlon. I did some hill climb bike races, and signed up for a few charity bike events.
Then comes the turn to triathlon, becoming hooked on the idea of it. This can happen through the nudging of a friend, making a promise impulsively while not thinking straight, watching the Kona recap from last year, or some other way. For me it was the Kona films.
The consequence of the previous step, the tipping point really, is to start practicing the triathlon discipline(s) that you are weak at. I had to learn to swim.
Then comes the plunge: Getting on some sort of a triathlon training plan, probably with a race already picked. It is a good idea to sign up for that race now. I signed up for IM France, nine months out, and started to take swim lessons, and got some organized triathlon training going. It was a homemade plan, that worked fine initially.
More about this last step next time, how to get training going.
So in short, a path can lead through all these gyrations I have recounted above. Or somebody may just sign up for a triathlon, then stand up from the couch and go find that old mountain bike in the basement. Whatever the path is, it is a path worth walking down on.