You do not state what country you are from but there are many roads that lead into CSI. My suggestion is that you do a Degree in Social Science (majoring in Criminology). This degree will give you major points if you want to enter Criminology via the police force. My daughter got Honours in Criminology and now works (as a civilian) attached to the police working on the statistical side of criminology on a national profiling data base (in Australia). She loves her job and it is one of the highest paid civilian careers within the police force.
If you want to work in forensics (eg as a forensic pathologist) then you will need to have a combination law/medical degree. It would be a great idea to speak to your Year 11 Careers Advisor (if you have one). If your school does not provide a Career Advisor, then your parents should seek one out (there are Career Advisors who provide the service privately). They will have all the intricate information you seek to work in the area of your choice.
Criminology is a vast employment area that encompasses a wide range and diversity of professions, from criminal psychology, psychiatry, counselling, medicine, police, facial reconstructors to name a few. Here is a link that will provide you with some great information: http://pattyinglishms.hubpages.com/hub/Careers-in-Criminology
What is important is that you aim to attain a tertiary degree in the field but be sure that the degree you attain majors in the particular criminology area you seek. If you join the police force, you can further your studies and become "specialised" in areas relating to criminology (should you desire to work in the statistical and policing area of the field).
In Australia, there is a huge Government-sponsored Institute of Criminology Unit which offers students a wide range of courses specifically focusing on professional careers within the field of criminology. Viz: http://www.aic.gov.au/criminal_justice_system/resources/aboutcriminology.aspx
If you are not from Australia, then find out if you have such an institution in your own country.