Most Helpful Guy
Just be yourself. Talk naturally like you normally would, and tell you read his profile, found him interesting, and perhaps even attractive, if you want to really set the hook. Many of us, myself included, love when the girl starts the conversation. It helps us know you're interested, rather than just being polite enough to respond to a note from us, if we were the first initiate. Pick out a few of the things about him that really resonate with you, and tell him you like that has those qualities or whatever they might be.
Most Helpful Girl
A guy will probably be pleasently surprised if a girl starts the convo cause most girls don't do that. Just say hey and then say something about something that he has written in his profile, find a common interest or something like that.