At least in the days before the Internet, the most common way of becoming trained was probably the same way I did it: I walked into the local dive shop and inquired about how to learn to scuba dive. I don't think I had ever heard of PADI or had any idea what a training "agency" was, let alone that there might be more than one such agency. All I knew was that that shop advertised "learn to dive" on a sign outside.I went through PADI because it was the only shop for 100+ miles. That is PADI's one big advantage-- it has the most shops worldwide. From what I read there is little difference with most agencies (NAUI folks will point out how their agency is superior.....). It's all the same stuff, just presented a little differently. Like beginning method books for learning a musical instrument.