Read the right rulebook
Identify the organization, class and current exercise list. Similar-sounding titles can have different requirements. Ask the organizer about eligibility and the practical arrangements for waiting with a large dog.
Keep practice short and clear
Work on an individual exercise in a familiar setting before combining it with others. Reward the response you want and leave room for breaks. AKC’s introductory guidance emphasizes practice between classes, with short sessions mixed with play and rewards.
Rehearse the whole visit
Travel, settling near other dogs and waiting your turn can be as important as the formal exercise. Discuss these parts with your instructor. If the environment is too difficult, a later event is a better goal than pushing through a stressful first attempt.
GO A LITTLE FURTHER
Sources & useful reading
AKC: getting started in obedienceA little more understanding. A better everyday life.