Regarding rules:
Most of the "spelled-out" rules can be found either in our general
Rules of Conduct, e.g. common-sense things like "not sexually explicit", "absolutely no dangerous items and nothing that looks like a weapon", etc.
Regarding the fursuit lounge, the most important rules can be found
here, or later posted at the entrance of the lounge.
Regarding "unwritten" rules, like "do-not-spoil-the-magic" (e.g. "don't run around headless in public, unless it's an emergency"), you'll probably find some panels about fursuiting pretty helpful. From last years, I would expect there to be something like "101 Fursuiting", and/or "Introduction to Fursuit Performance", but the program is not final yet, so there might be different ones.
Other than that: Watch and learn
There are literally hundreds of fursuiters at EF, some first-timers as well as some who built their first own suit around EF3. Many of them will be happy to share their experience if you ask them nicely, while relaxing in the lounge, at the bars, or all the other places where people at EF come together to chat.