+1 to all of what Jeff said. Also, don't wear any cologne. Put together a small judging kit with comfortable (padded grip) mechanical pencils, flashlight, corkscrew (stewards will have bottle openers, but watch 'em scramble if you come across a capped & corked bottle), etc.
Biggest thing for a new judge (or all of us) is to give solid, complete feedback. Use the score sheet as your guide & touch on ALL the elements mentioned. You don't need 'technical' descriptors, simple everyday language is great. Just describe your perceptions as completely as possible. If you feel confident in offering constructive criticism, please do so (regular posting to this forum is great experience for that). Don't make assumptions about what the brewer did, but offer possible actions to take in improving recipe or dealing with perceived problems.
Don't stress about preparation. Judging is fun, ask the organizer to pair you with experienced judges (especially if you already know them) though they usually try to do that anyway.
Good luck & have fun.