| 18 years?? You're a light weight. Try going 24 years before you kiss a girl, Ha ha ha.. Still, even I probably don't hold the record..and I'm not a big nerd either, and I'd like to think I'm not too shy, either, but I had the same problem. In a situation where the mood was right, I was (and sometimes still am) hesitant because I'm afraid that the girl really doesn't want to cross that line.
Bottom line, you have to weight the risk that she doesn't want to kiss you back and what her reaction might be. Honestly, the biggest hurdle is your own shyness to just do it. In most cases if it seems right, it probably is right and the girl will be cool with it, maybe she even wants it. And in most cases the worst case is she'll back away when you move in for the kiss. It's not a big possibility that you'll fumble things badly enough to loose her as a friend. Unless you're a real jerk, overly insistant or what not, the worst case is things will get weird for a little while..but like I said, chances are probably low if you're feeling the vibe, and you are your biggest obstacle holding you back.
I agree that it's best not to have a discussion about it.
See, like you just answered yourself, most of these matters are common sense for the most part and what you read into each individual situation. They're all different, but there are a lot of behaviors that a vast majority of people follow and become conventional signs that allows you to read what's going through the other's head.
If you really want to you can search for similar posts in this forum becase I know there's been some good advice in the past...but like I said..you can probably figure a lot of it out for yourself.
Freddie
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All I know is I know nothing - Socrates | 
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