Once you have created an account and joined a department, sign up for an interview.
Look under, 'Interviews and Training Sessions' in the Opportunities tab. Or you can also see a complete list of interviews for all departments in 'Public Opportunities'.
You are able to join an interview session in any department, with the exception of Aquatics, which has an exclusive interview and training process that involves being in the pool.
You will need only one interview, no matter how many departments you join.
You may interview again if you would like to change your ranking, after getting volunteer experience over a season or more.