1) be yourself. You're probably a lot cooler than you give yourself credit for and genuine responses always go over better than contrived nonsense.
2) Say "hi." You'd be amazed how far that can take a conversation.
True story: Not long ago at Boston ComiCon, I ran across a lady who had read all four my Seven Forges books. She had no idea I was going to be there and when she realized who I was, she freaked a little. Believe me, it's very flattering. It was also cool to just talk with her and calm her down and discuss what she liked about the books.
As I had no copies of THE LAST SACRIFICE with me, I gave her a postcard with thew cover on the front and the link to the Angry Robot website page on the back. She had all of my books on Kindle, but she bought them again so I could sign them.
Put a smile on my face that lasted the whole day.
3) Relax. With VERY few exceptions, (they do exist) writers are delighted to meet you. I always am. Maybe not in the bathroom at the local convention or anything but still, delighted to meet a fan of my work or even someone who is curious about it.
4) Bonus round: Be friendly and be polite. Happily it hasn't happened to me (YET) but there are a few people who've had "fans" come up and be nasty about a book they didn't like or understand. Never gonna be my favorite thing.
But especially remember rule number 2. I can't have a conversation with you if you don't engage and I'm looking forward to meeting you. It's why I attend conventions, etc.
In the meantime, look at my cool new cover!!!!
