The path to become a forensic pathologist is pretty long after high school. You begin by getting your high school degree and getting into college.
In college, a few things are at play. You need to take a core set of classes, usually including general chemistry, organic chemistry, physics, calculus, statistics, biochemistry, biology, and writing, and you need to do well in them (try as hard as you can to get A's in the sciences at least). Now, you need to also take the Medical College Admissions Test (MCAT). Most people, along with their college courses, end up taking review courses for the MCAT to ensure a high score, which is necessary with your good grades for the next step: getting into medical school.
Take it from me (current medical student) that getting into medical school is extremely tough, and getting through the coursework is much more difficult than college. However, after 4 years, you will have your MD or DO. During medical school, you need to have decent grades and a decent USMLE Step I and Step II scores to get into a pathology residency after getting your degree.
In your residency, you will learn anatomical and clinical pathology over 4 years in order to get board-certified. You may want to do some research in your area of interest to make your future fellowship application more attractive.
After four years of residency, you will then apply for a 1-year fellowship for forensic pathology, and if you get in, you can be board-certified after you have completed the work!
So, to summarize: 4 years college + 4 years medical school + 4 years residency + 1 year fellowship = 13 years of post-high-school training.
Hope that helps! See the link below for more info on the training for this particular career!