Mine was Penny Arcade, waaaaay back in their earlier days. Like, probably back in 2000 or so. I was 12-13, and got to borrow the family computer for half an hour every afternoon. My brother - who knew I liked drawing, and knew I liked video games - told me I should check out Penny Arcade, so I did. And it's all been downhill from there, hahaha!
I don't read it any more - I stopped years ago, in fact - but that's where I started.
.... Goodness me, I've been reading comics on the internet for 15 years. D: