You call your local job centre to get an appointment.
In the meantime, if you need a temporary number it's simply TN + DDMMYY (your birthday) + M or F (male/female).
So, a male born on December 15, 1970 would be TN151270M.
For some reason they don't record your name with a temporary number. When I went down he just said to use it. I don't know how it's actually tied to your name then but that's what they did with me.