Smile, relax and be yourself. Be prepared to ask and answer the standard questions:
What's your name?
What you working on?
Why are you here?
How long have you been coding, learning, etc?
What are you looking to get out of the Meetup?
Do you know of any meetups?
Do you have a business card or how can I get in contact?
Conversation will usually flow around the above. Usually the setting is quite formal and don't be scared to interrupt and introduce yourself. Good luck.