How to measure for a new bra?

[ONO7]

Having lived in those comfort bra type tops for a few years, I have decided I would like a proper bra.

I have no idea how to measure for this, any advice would be really appreciated.

Would need to order online.

Thanks.

Use the Boob or Bust method, here: www.booborbust.com/

Thanks so much @violetsanddaisies

Well booborbust was an eye opener. I would have thought I was a 36C but apparently I am a 34G.