When (and how) did you become certain of your sexuality?

So I'm an almost 15y/o boy. I'm sure I'm LGBT but just not sure if I'm bi or gay. I am attracted to men and the male body much more obviously than women (I.e. I can easily get off from men) but I've never had a full on crush on another boy before. So, there's a girl who I've been friends with all my life and we talk on a regular basis who I would be happy to be "dating" (whatever that means at my age) but I don't know if I'd kiss her or something like that so she's making me really confused. I don't think I'm stereotypically gay if that is anything to go off on...(i.e. I hate shopping, I don't have an extremely camp voice (some say I have one slightly, others don't). I have a few female friends but that's about it. So my question is, when should I expect to know? And will I eventually figure it out or is there a method to help me figure out (not doing anything sexual btw)? Thanks :)