Meet the Volunteer Coordinator there, they are very nice and will ask you what role you are interested in. Most of the time the coordinator introduces the roles available to you. Almost all departments need volunteers. You can do some research first, there are brief role introductions for every role when you fill in the application online. Think of 1 or 2 roles before meeting with the coordinator and let her know during the interview. During the interview, she will also show introduce you to meet certain departments when she thinks you are suitable.