Here's my attempt to kick off a new topic! Apologies if it's been done before (I tried a few searches which suggested not, but the search facility is very limited).
What do you remember as your first experience of getting turned on in real life (as opposed to, say, whilst experimenting alone during puberty)?
In my case, I was at school in my mid-teens. I was eating lunch on my own in the canteen and I exchanged glances with a girl on another table whom I vaguely knew and quite liked, though without any sexual element (or so I thought). Despite being very shy and guarded at that age, I must have had a positive expression on my face, because she flashed a lovely warm smile back at me. It caused me to melt inside in a gorgeous way. I've always been a sucker for lovely warm smiles, even in a non-sexual context. As I looked away bashfully, though, I became aware that the table at which I was sitting was saving my graces by concealing a tell-tale bulge in my trousers.
It was a wonderful experience, largely because it was so unexpected but (fortunately) under control. Nothing further happened with the girl, but I owe her a debt of gratitude for putting me in touch with my developing sexuality, even though that probably wasn't her intention at all.