To join the police you will have to apply to your chosen police force. There are no formal qualifications required but you will face a complex application which asks for four essays detailing situations you have dealt with in your past which reflect core competencies. Should you pass this stage you go on to an assessment. The assessment comprises a written test, a maths test, two further written assessments and four role-plays, plus an interview.
Should you be successful, some of the larger forces have a second interview. Then there's a medical and fitness test.
If you pass all those you should be offered a post of trainee police officer. The training takes around 10 months and the probationary period is two years.
After that you can apply to CID.
Think carefully and if it is your choice of career, all the best. Oh and be prepared to deal with people who perceive you as a brutal lowlife one day, useless the next and then expect you to defend them the day after, based on media hype and the actions of the few.
Best of luck.