Rules re: involvement depend on your school and the service. When I take students I am told they aren't allowed patient contact. Whether that happens or not I cannot divulge. The only suggestion I have for you is to ask many questions. Try to defer your questions until after the call. When checking out the truck, ask what things are, how/when they are used, etc. It is likely some questions will be thrown back at you, so be prepared to do some thinking.
It's likely part of your time will be spent in pure boredom, so consider bringing a book (EMS related?), or find ways to learn when posted... Think ahead about different types of calls-- what will you take, why?