I don't consider myself a rationalist, I feel that would indicate a confidence I don't have. I'm certainly trying, which is by far the most important step I think.
I suppose it was the first time I heard to proper definition of evidence, in that anything that is true only ever makes anything else more likely to be true if the latter has a higher chance of being true if the former also is (dumbed down but that's how I heard it). Id always been a bit skeptical of - effectively stated - all the bullshit but that's what really got me thinking about religion and some of the things my parents taught me.
I believe I was 10 or so. I was mildly astonished at the fact no one considered this relevant information until now. I'd been in school for 4 years and no one taught me the concept of evidence.
The depths of my naivety are revealed to me now.
To break up the awkward silence at the start of a recent Overcoming Bias meetup, I asked everyone present to tell their rationalist origin story - a key event or fact that played a role in their first beginning to aspire to rationality. This worked surprisingly well (and I would recommend it for future meetups).
I think I've already told enough of my own origin story on Overcoming Bias: how I was digging in my parents' yard as a kid and found a tarnished silver amulet inscribed with Bayes's Theorem, and how I wore it to bed that night and dreamed of a woman in white, holding an ancient leather-bound book called Judgment Under Uncertainty: Heuristics and Biases (eds. D. Kahneman, P. Slovic, and A. Tversky, 1982)... but there's no need to go into that again.
So, seriously... how did you originally go down that road?
Added: For some odd reason, many of the commenters here seem to have had a single experience in common - namely, at some point, encountering Overcoming Bias... But I'm especially interested in what it takes to get the transition started - crossing the first divide. This would be very valuable knowledge if it can be generalized. If that did happen at OB, please try to specify what was the crucial "Aha!" insight (down to the specific post if possible).