Yoy have NOT said which part of the Country you live in and where you want to work !!! ??? It's a big place!!
London: If you are looking at London, you have to pass something called the "MET Test". This is a translation test to ensure you are up to the standard the Police require. If you contact the Metropolitan Police @ New Scotland Yard and ask for the department which deals with translators, they will give you the name of the company which sets this test. I think there is a registration fee.
You can't "join the police force" unless you intend becoming a police officer. You can apply to work FOR the police as a translator.
As TheTrog says, you haven't specified where you live. Also, you haven't specified which language you can translate. For example, it would be pointless applying to work in the Orkneys if your specialist language is Swahili.
Either give us enough info to help you or, as Rev says, show some initiative.