There's really only 4 things expected of judges
Be on time (the coaches will let you know what time to be there)
Listen intently to the performing students. Don't get distracted, be on your phone, or anything like that.
Keep an open mind (don't let your own opinion get in the way of being an effective judge)
Give some feedback. Sometimes the feedback is two sentences, and other times it can be 2 paragraphs. Each judge takes a different approach, so find out what works for you, and go with it.
Not at all! There will always be a FHS coach at the tournament to walk you through any event, how to judge it, and what to look for. Typically, there are also several veteran judges available that can walk you through events as well.