Sheep are tagged at birth with the flock number of the holding where they were born. So any sheep you buy in will already be tagged and you do not apply your own tags to them.
If and when you have lambs born in your own holding, then you would put your own tags in their ears.
As soon as you have land on which you intend to keep sheep, you apply for a holding number. This has the format cc/ppp/nnnn, where cc is the county code, ppp the parish code and nnnn the individual holding number within the parish. You cannot move sheep onto your land until you have a holding number.
You would also tell Animal Health that you intend to keep sheep, whereupon they will issue you with a flock number. This has the format UKnnnnnn. When you have in-lamb ewes, you would then order ear tags for the lambs to come. These tags would have the flock number on them.