Conventions are normally very crouded, and a bit of a culture shock for first timers. Most of the people who go are really nice and willing to help newbs (no offense). If you get lost or need help just ask someone and more often than not you'll get help.
Actually the most popular costumes are homemade, especially if they are characters that not a lot of people can pull off. For example I made my own costume for Ryuk from Death Note, and everyone loved it.