How can I express attraction?

This is an awkward question for me to ask, but I'm no longer a teenager and I've never had a girlfriend or kissed a girl. The only girl I ever asked out (and for whom I ever really felt what might have been love for) said no, though admittedly I did not know her that well. I have been worried for a while now that I will die alone (at my present success rate, I'll die without even having kissed). I'm not asexual.

I want to change this, but I just simply don't have a clue. I used to have a passive attitude and hope that I would just happily fall into a relationship, but now I realise that doesn't work. Frankly, I don't even know if I want a relationship; maybe I just want to feel like someone cares for me and is attracted to me, or I just want to confirm myself that having a relationship is still possible.

I was depressed, shy and focussed on other things at that age when people become interested in the other gender, so I didn't go out much and completely missed out on the social training that comes with that stage of life. I'm not so shy anymore, and don't have any problems talking with women I know.

So, basically, is there a way to learn from the beginning? Are there online guides that might be useful? How would I express attraction to a friend, and how would I express attraction to a stranger in a library or nightclub?