Approaching someone online isn't easy - at least not the first billion times. But by the billionth + 1, one gets pretty comfortable.
Until then, it helps to employ a little strategy. Some common sense also doesn't hurt.
I like to think of the first hello as approaching someone in a bar or at a party. That's a good strategy. After all, he is on a dating site. Clearly he's looking to meet people for some reason.
But at a party, I could just walk up and say hi.* Then the conversation can flow naturally (Do you like this place? How do you know so-and-so? What are you drinking/eating? Whatever.)
That doesn't really work online - and that's where common sense needs to take over. If you email me a single word ("hi"), what am I supposed to do with that? Forget just saying "hi" back - it could be hours before I reply. Exchanging one sentence at a time over the course of a few days is tiresome and boring - and no way to meet new people.
Your email needs to have at least a little substance. Say something that can spark a little more conversation. Ask an open question (or maybe two). Extend a compliment.
And for goodness sake, use correct punctuation and grammar. At least make a solid attempt.
Another thing about the opening line... At a party or club, you may be able to just rely on your looks. Hey - I'm willing to admit you might just be that good looking. But no matter how attractive you are (or think you are), it won't translate online. I may not be able to see your picture, and I definitely can't be sure that it's really you. Plus, in person, you can make eye-contact and see if there's any attraction. That won't work online, and assuming that I will be attracted to you is just...well - unattractive.
So, say hi, and then maybe say a little more. Give a person something to work with.
*Hypothetically, of course. I have never had the courage to actually do that...I'm just pointing out the option exists.