Junior golf tournaments run differently from most other youth sports events in one significant way: the players are responsible for themselves. They keep score, they know the rules, they call penalties on themselves, and they deal with the decisions on the course without a coach on the sideline. That is part of what golf teaches, and it is part of what makes it hard on parents who are used to being able to do something.
Most junior tournaments use stroke play format: total number of strokes over the round determines placement. Match play, where players compete hole-by-hole against a single opponent, is used in some formats including the USGA junior events. Knowing which format before you show up matters.
Scorekeeping is handled by the players themselves, typically with another player in the group serving as the scorer. At the end of the round, scorecards are signed and submitted. An incorrect scorecard, even if the error was accidental, results in disqualification at most events. Before the first tournament, your player should know how to keep score accurately and how to mark their ball.
Tournament entry is typically handled online through the organizing body (USGA, state golf association, AJGA, local club). AJGA events require an AJGA membership and performance criteria at higher levels. Local and state association events are more open and a good starting point for new competitors.
Parent rules during a round vary by tournament. Some events prohibit any parent contact with the player during the round, including providing distances, selecting clubs, or giving advice on anything other than the rules of golf. Other events allow parents to walk the course as observers but not coaches. Know the specific rules before the day.
What to do when something goes wrong on the course: let your kid handle it. If there is a rules question, the player asks for a ruling from an official or figures it out with their playing partners. The parent on the course calling out distance numbers or suggesting club selection is interfering with the competition and, depending on the event, can cause a penalty.
First tournament expectations: most kids play over their handicap in the first event. The nerves, the unfamiliar course, and the formality of competition scoring all push scores up. This is normal. Focus the debrief on what the experience taught them, not where they finished.