Do you mean to be the fella/lady who autopsies people and weighs their liver and brain and what not looking for a cause of death? This, I think, is the 11-12 year (medical Doctor) forensics profession.
If you mean to be the C.S.I. forensics collection and analysis (fingerprints, DNA, blood splatter) kind of fella/lady then you don't need the 11-12 year medical Dr. degree. I think they have very specific programs for this kind of training that are 4 years or less. But, I'm sure a PhD would land you at the top of some major city's forensics department. Actually, I think the lead forensics guy in the OJ Simpson case was a PhD... what was his name... the guy who messed up the chain of custody of OJ's blood. I can't remember, but I do remember being in middle school at the time thinking that would be a cool job.
In either case- AP science, chemistry, biology and math
good luck
-modest