It can honestly depend on what con you go to. My first convention was San Diego Comic Con way back in 2004 (It was a lot different back then, but still HUGE)
I agree with DX, you should pre-register if at all possible. It saves both money and time.
What else can I cover that hasn't been said? Oh. People.
You're going to meet all kinds of people. Some of them will may be very different from what you are used to. So, I guess the best advice I could give you is to try and keep an open mind. Who knows? You might make a few new friends in the process!
And don't worry about not cosplaying! Just go to your Con, grab a schedule and have a blast!
Oh, and if you can, bring your own food and save a little money!