I would like to offer a suggestion on shifting your question. Instead of asking “first fap”, consider asking things like “at what age did you start” or “how did you first come across it?”.
Even these questions that I suggested are still susceptible to a problem: talking about triggers and visiting those memories can be a trigger.
The important thing to focus on is what you touched on in the second paragraph, how to identify and avoid these “distractions and troubles”.
Anyways, those are just my thoughts.
To answer your question, the way I got into this PMO addiction was simply by looking for too long. Inappropriate images are everywhere, and when I saw them I would stare at them. Eventually that led to me searching for them.
There is no way to avoid that, so what would have helped, that I am still trying to do, is act IMMEDIATELY and look away, think of something else.