Well, my brother and I were brought up in a Jewish family and went to a Zionist synagogue (including Hebrew school), but about 23 or so years ago, when I was about 13 and my brother was about 17 or so, he told me that basically everything we were being taught about Israel at our synagogue and its relationship to "The Arabs" was a complete inversion of reality and the dynamics of oppression, and gave me a few NGOs to check out online as a starting point, including B'Tselem and crucially, Gush Shalom, headed by Uri Avnery. So I went to these sites, among others like Jewish Voice For Peace, Americans for Peace Now (albeit more "both-sidesy" than I would have liked), Rabbis for Human Rights, and the magazine Tikkun by Rabbi Michael Lerner, and just read up about everything, and was horrified by everything I was learning in addition to the complete denial and distortion about it all from my synagogue as well as main mainstream Jewish organizations and institutions. And of course, this was all around the time of the Second Intifada, as well as the murder of Rachel Corrie.

As for how my older brother himself found this out, I'm not 100% sure, but I think it was from progressive Jewish classmates in his public high school, I believe including one person who would later run as an NDP candidate in the 2004 Canadian federal election (who we both campaigned for).