Different countries may have differing requirements for becoming a CSI. I would start with a 4-year degree in one of the following fields: criminal justice, forensic science, biology, or chemistry. Advanced computer skills will add to your talents.
You will need clear, high-quality communications skills, including verbal, written, and online presenting; along with exceptional attention to detail and an awareness of your own biases in the investigation. Problem-solving skills and critical thinking are imperative, and you must always remember that humans tend to connect things even when they are not connected; so, be aware of that bias.
Collect some law enforcement experiences. If you have none, begin by seeking ride-along with your local police force. Try the local police department's Citizen's Academy that provides some training for non-police. Become a police officer or security guard and seek extra training. If you are in the military, seek military police training.
In spite of all this, some people serve as private detectives (license required) or store detectives (training required) and find this helpful instead of becoming a police officer.
Whatever you decide is your path, follow it and then apply for CSI positions and access these sites for interview advice: https://hubpages.com/business/Employee_Qualities,