Usually the PT you are observing/volunteering for will ask you to help out, whether it be to stretch a patient, put away some equipment like a theraband, or play catch with a patient while the PT is guarding them. At first you might do more observing, and then as time goes on you'll help out and get some hands-on work. You may start off getting to help out right away. It all depends on the PT you're with. That being said, it's very helpful to ask questions, just not too many that would distract the PT working during the session. A good PT will try to explain some things that he/she is doing or about the pt's diagnosis/background (with the pt's permission), but don't be afraid to ask questions if you have any.
I remember asking the PT I volunteered for what spinal stenosis was, as I had never heard about it before, or why she had a patient with carpal tunnel syndrome doing a wrist flexion/extension exercise. Asking questions made for an engaging learning experience. During school I often reflected back on what I remembered during my volunteer days and what the PT did, which helped me better understand and apply what I was learning at the time.
Another note, is that even though you are interested in this field, at times you may feel very tired, because you're standing there and watching instead of doing all the work the PT is doing. So don't feel bad if you feel 'bored' or 'restless'. Asking questions and trying to stay engaged is the key. Good luck!