I think I was unknowingly just over the horizon from apostasy for a long time. I think if somebody, certainly future me, would have sat down and asked me a few good questions I would have left christianity much sooner. I think a lot of the problems I had were subconscious, so I didn't really see a problem at the time. There was nothing to compare my life to, and I thought all the confusion I was experiencing was just part of growing up, all that adolescent stuff.
I would have told myself to read The Demon Haunted World. That book was perfectly timed for my teen years, but my parents said Carl Sagan was a satanist and I just accepted their word. (I didn't read that book until last year, mid-30's, half a life later than I should have.)
I would have asked young me why my parents were so no-premarital-sex and no-drugs, but yet they loved music like all the old hippie music and even old jazz that talks about drugs and sex.
I would have told young me to seriously look at evolution.
If possible, I would have given younger me a copy of The Skeptics Annotated Bible. Even if not, I would have pointed out all the awful things in the OT and all the inconsistencies and urged young me to actually read the bible completely. (Somehow I got through my christian life without ever reading the whole bible, or probably even half of it. This is partly because school destroyed reading for me, but partly because I was already so disinterested in it, thus my first statement that I was probably just over the horizon from apostasy.)
It probably would not have taken much to get me to leave christianity. I think this is one reason that people who are atheists shouldn't just sit to the side and keep their mouth shut. I think atheists should ask serious questions and respectfully challenge christians to find The Truth.