I read Catch 22 when I was about 14 or so - it was literally the first "grown up" novel that I read that wasn't dry science fiction and it completely enthralled me.

What did I learn from it? Possibly this:

"The enemy is anybody who's going to get you killed, no matter which side he's on"

A favourite of mine as well. I read it when I was about 20 and it blew my mind. I later read other books by Joseph Heller but I really shouldn't have bothered. Catch 22 was really his main opus.

I could never get past the first chapter, The Soldier in White. Somehow I took it too personally, or realistically, or something, and I didn't want to continue this journey.

My choice as well- it was the first book to show me that societies are systems with all the baggage that entails.