Forensic psychology degrees focus heavily on psychology, sociology and science courses. Take advantage of any freedom in course selection your high school allows to take as many of these classes as possible. If your school offers AP (Advanced Placement) classes, take as many as you can. Not only do AP classes improve your transcript, but scoring high on an AP exam will help you earn college credit and cut down on the time it takes to earn your degree.
As an undergrad you likely will be working towards a degree in psychology. Although not all master's or doctorate programs require a bachelor's in psychology (some may just require a few courses), it certainly would not hurt your cause. Also consider taking classes in criminal law and sociology to improve your understanding of these fields. Some universities offer a bachelor's in psychology specializing in forensic psychology, which would serve as a smart precursor to your master's or doctorate in forensic psychology.
You will need a master's degree--and preferably a doctorate--to have a successful forensic psychology career. With only a bachelor's degree your job and salary opportunities will be extremely limited. Getting your master's degree will take two years, and getting your doctorate could take an additional two to three years depending on which program you choose.
As many programs do not require a bachelor's in psychology, you also can use this route to start forensic psychology as a second career. You will likely need to take a few prerequisite psychology courses before applying.
You will need to spend a few years working as a clinical psychologist before moving on to forensic psychology. You must learn to understand treating "normal" people before advancing to forensic psychology so you can comprehend the difference between working with criminals and noncriminals.
Forensic psychologists require constant on-the-job training including attending conferences and seminars to learn about new case studies, research and techniques related to forensic psychology and stay up-to-date and efficient at their job.