Growing up in L.A., Reed had a thing for horses. ''My grandma breeds them, so I was really into horseback riding,'' says the 15-year-old actress. ''I used to go every week.'' Then junior high happened: loneliness, jealousy, self-destruction. Reed coped by pouring her heart into a screenplay for Thirteen, the fast-living adolescent drama she costars in with Evan Rachel Wood and Holly Hunter (see review on page 54). ''I knew nothing about writing,'' says Reed, who shares an author credit with director/friend Catherine Hardwicke, 48. ''But Catherine wasn't 13 and I was, so that's where the movie got its basic outline. It's my voice.'' Which didn't get her out of auditioning for the role of manipulative, troubled Evie -- especially since her resume consisted of a school play in kindergarten. In the next five years Reed is looking forward to college, and maybe even more acting. ''I don't know where my life is headed.''