well I am only 17, and I have meet many girls online as well a in real-life situations, well I alot for a 17 year old anyway, and the easiet thing, if you find them off a dating site or something is to just say
Hey whats up, my name (insert) and I saw you on somesite.com, and I thought you seemed interesting and I might want to get to know more about you and see where things go from there.
This is simple and it doesnt show too much interest, so your not all over, you put "might" want to get to know more, so it gives the the idea that she better sound interesting and keep your attention or your gone. I also dont like to put anything to do with their looks, because these women normally come on the sites because guys have used them before, and also because it gives them to much initial power in the relationship.
If its a situation where you want her phone number, screen name, e-mail, or whatever then just add something flirty like "well I cant talk to you until you give me your screen name, and then I send you a message sometime. this commands a confident tone that will almost make her give you her contact number of name and it also doesnt show you as needy becuase you said you would contact her sometime, and then my advice is to wait 2 or 3 days to contact her, just like meeting a girl in a club.
Word of advice though is that you will get many rejections compared to few hits, but the good news is, that there is a far smaller fear of rejection, as well as the fact that girls are naturally cautious. But I do have a friend that is 17 and met up with at least a 9 just for sex. Also I recommend as soon as possible get her phone number because she will be more likely to go on a date, rather than her talking herself out of it cause she has only talked to you online. As always try to get them to contact you on the phone or through e-mail, otherwise some of these girls may not get much attention, and then as soon as you give them some, they get very arrogant and wont talk to you until you call them or e-mail them.