hello, everyone

I would like to apply the national insurance number but I don't know how to do. I came from China with a work permit as a highly skilled migrant three months ago. I help another person who is director set up a limited company two months ago as a secretary and I am responsible for daily work of the company now. What documents I should submit if I would like to apply for the national insurance number. Do I need to have a employer letter or contract? I look forward to hearing from you soon. Thank you very much.