Becoming self emolyed

I have been working in a salon for around a year and have built up a steady clientele but recently the salon owner has told us we have to go self employed.  Where do i stand with this? can he just say we have to do this with out any notice or anything.  im not agianst going self employed but im just worried im not busy enough and also by boss does not seem to have clear facts for us all.

How do i pay national insurance?

He wants us to buy colours from him at an inflated price is this normal?

we have to buy our own card machines to process payments is that normal?

I have to pay for the use of the receptionist when she does not work the same days as me also juniors are not always in so why should i pay for their help too?

would i normally invoice my boss monthly to get paid?

its confusing!