Say hi and give them a big, natural smile and hug immediately. It gets a possibly very awkward part out of the way and puts you both more at ease. If you wait to hug, it can make it very awkward later. Just remember that in the grand scheme of things, you are both just two ordinary people. They are just as nervous as you so try to just relax. Imagine that you're greeting a friend you've had for many years. I hope that helps a little. I've been there too.