Depends a bit on where you are.
In Scotland, you will register with scoteid where movements are recorded online.
The seller will notify the movement and you log in to confirm receipt.
If under 12months and not registered, they will likely be moved with a spray paint mark for identification. If over 12 months or registered, they will have an ear tag relating to their original holding.
If you then move them to slaughter, you need to apply your own tags (with the herd number animal health provide you when you tell them you intend to keep pigs) or slapmark.
You need to record in your holding register a tally of pigs moving on and off, born or died. Again, animal health should provide you with a book for recording this.
Any questions, call your local animal health (animal and plant health agency or apha in Scotland) office.
If in England, I'm sure someone will be on soon who can give you the right details of how it works there.