A forensic psychologist is any psychologist who offers an expert psychological opinion in a way that it impacts one of the adversarial arenas, typically the courts.
Many people think of forensic psychologists as focussing on criminal matters.
This is certainly not always the case. A forensic psychologist is any psychologist who by virtue of training or experience may assist a court or other fact finder in arriving at a just or correct decision. For example a forensic psychologist may assist in a wide variety of civil matters. It will probably take you about seven years after the undergraduate degree to get this doctorate, though some people may take a little less time. One way the person who is already a psychologist becomes a forensic psychologist is by doing additional informal study after receiving the doctoral degree.