Don't worry about it at all. I was scared to death when I attended my first Costume Con but quickly found out there was nothing to worry about at all. The people are friendly, very interested in costumes and are, in general, very accepting of anyone in any type of costume. You'll learn tons at the panels and have a great time I'm sure. You don't have to compete or anything just enjoy the costumes around you and ask questions if you've a mind too. The people won't bite and they'll be happy to answer your questions.
Conventions 2014: Costume Con, Superman Celebration, Project Comic Con. (Others TBD)
Working on: Air Nomads- Lion Turtle City of Air-LoK, Lily Potter- HP, Fire Lord Zuko- Older version-AtLA