Applying for an agent... definition of "experience"?

Hello Everyone, This is not your usual "how do you get an agent post" I have a more specific inquiry. I'm 18 and about to graduate high school in December. I've done alot of research into the type of agent that I'd like (I want to work in film preferably), following the guide you offer at this website. So I called them up and asked how to be considered to be represented by them. However all of them said I need more experience to be take on board. So my question is, what kind of experience are they looking for? I want to do a bachelor of music, so I can't go to acting school. I've been extras, been in the Logies... but do they want someone who has already had a leading role on TV? If so, how is the average teenager supposed to secure that kind of role without an agent to begin with. Any help would be appreciated, ~Ayane