The Police is a whitelisted faction which means you have to apply and work to get in. First you have to have your Application accepted (being over 16 is a requirement but you might be put on young recruits if you impress in your application) after that you get an interview on teamspeak where you're asked a series of questions, if you pass that you go on to doing your FTC test and if you pass that you can become a CSO which is considered a training rank, it'll be your first 2 days in the police force where you won't be given any form of weapons and instead your primary focus is going to be learning from higher ups and grasping the bases of becoming a good officer, after 2 days have passed you will be eligible to do your FTT (Field training test) where you go through a series of tests where academy members evaluate you on your skills and abilities in a series of different scenarios. If you pass that you become a PCSO where you're still considered in training but you'll be given a taser and a rubber bullet gun, after 5 days you're eligible to go for your PA (Practical assesment) where you lead a patrol with higher ups from the academy where you get to boss them around, give them roles and such. If you pass that you'll be granted the rank of PC and you'll have the option to choose which constabulary you wanna join.
Hope this helped you out.
(I'm sorry if I mentioned anything here about the Police training I wasn't allowed to mention on the forums, feel free to remove my comment if I did.)