You apply for the NHS by going onto the CMG forums here: https://cmgstudios.net/forums/index.php?/forms/6-nhs-application-form/
You can find this out at the bottom of the command structure page.
You can find out what uniform you need on navigation bar go to NHS Procedures then navigate to Uniforms and then locate your rank within this page.
You can gain promotions through high levels of activity and being recognized for extremely good roleplay. You can do this by showcasing what you do in #screenshots-videos, recieving praises in the Discord and by having high hours. As High Command, we regularly check people's time to ensure promotions and rewards for good activity are handed out frequently.