If she has a profile, read her "about me" section and see if there's something you can mention or make a joke about. A common interest, her hobby etc. Make it as personal, interesting and funny as possible to differentiate yourself from all the other guys messaging her. Trust me, most guys start a conversation with hey, what's up and that's just so boring... Imagine getting about a hundred messages like that everyday. After a while you just stop responding altogether because you simply get too many messages. Besides, it seems like most guys just copy&paste the same message and send it to as many girls as possible. It's very impersonal. Personally, I appreciate when he actually takes the time to read my profile and sends a more personal message. It definitely stands out and I appreciate the effort so I'm a lot more likely to respond. I'm not saying that hey, what's up doesn't work but in most cases, the girl has to find you attractive from the start
Be minus the "you want to date" part. You're on a dating site to connect and date. Assume that's what she's there for too until interaction proves otherwise. (i. e she avoids meeting up after you've spent some time chatting)
use a cheesy pick up line as a joke, anything that could make her laugh or start a conversation is good. Also, try and use 'how was your day' as opposed to like hru or something short like that ((: (worst for me is 'sup' as a first message)
As far as I have observed the mating rituals of women, it would seem that making light of something, perhaps her, and acting indifferent to whether you get to copulate with her seem to be the best way to go. Just try and hide your desperation and deep feelings of inadequacy, as much as possible, is the way I usually go about it.