Whoever you are getting them from needs to tag them with the tags for his or her holding. One electronic (yellow) tag and one non-electronic, one in each ear, both with the flock number and the same individual number on them.
You may be told that they only need one single electronic (slaughter) tag. A slaughter tag has no visible individual number, just the flock number, although the individual number is electronically readable.
In practise, if you plan to eat them yourselves or sell them before they are 12 months old, the single slaughter tag will probably suffice. But the legislation calls for two tags, showing the individual number, for any animal that is leaving its holding of birth for other than a slaughter market or the abattoir. So it's safest to insist on this.