As a man, the only good romantic relationship I've ever had is with a woman who was my friend for over a year first. This is entirely because it showed we had the same interests, similar opinions, and could consistently relate to one another; the people who say opposites attract can simply go fuck themselves. The girls I dated before my current girlfriend were pretty, had superficially similar interests (I met one at a con, for instance), but I eventually got bored with them because they were asinine and their powerlevels were far too low. I just say this because I personally have never been in and cannot imagine a long-lasting relationship based entirely on flirting.
This is a pretty fucking obvious statement, but male/female interpersonal relationships are a little bit awkward on account of our biology. Although I wouldn't go as far as to say "men can't have female friends," since that sentiment is utterly fucking retarded, when a man is single and a woman is single and neither thinks the other is fugly, no matter the context, it's impossible to escape from some degree of sexual tension. Generally speaking, this tension is absolutely meaningless and goes unsaid for that reason (or, if one of them is already in a relationship, the feeling will get shot down by the thought "I don't need them, I already have someone I love," unless they're a rotten person). With that in mind, when you meet a single guy who you share a common interest strong enough to have an actually significant (either talking in depth or an hour or more long) conversation with and he's friendly with you, you've got a chance with him - you can capitalize on said sexual tension.
So, here's something fun that I noticed.
>teenager
>never had a boyfriend
>shy, kind of introverted
>sarcastic
Did you know that those are actually considered adorable by many and if a guy is already noticeably flirting with you, you're attractive enough to warrant that guy's attention at all and thus have an opportunity to use these cute qualities to your advantage? All you have to do is be honest. I know that's hard, I know you fear rejection - everybody does. But at the point where he's flirting with you, all you have to do is literally tell him he's cute, touch his hand or arm, and be bashful. You don't have to be forward, all you have to do is reciprocate and be a little affectionate - be as affectionate as you want to be, in fact. Hell, if he's flirting with you, be as affectionate as you *wish* you could be.
Otherwise, >>65580's advice is pretty sound, if a bit laughable in that it characterizes men as "opportunistic wolves" whose subconscious minds you want to exploit. Well, I guess us guys often do the same shit when we trade tips about approaching women. What gets lost in the rhetoric is that men and women are just people. There's nothing particularly special, mysterious, or significant about them in the grand scheme of things.