You will need your insurance certificate, the renewal notice from DVLA or an appliction form from the post office, the MOT if the bike is over 3 years old, the payment.
If you have the V11 Tax Reminder, you need to take that together with the payment, your original insurance certificate and, if applicable, a current test certificate.
If you don't have the Tax Reminder, you need to take the V5C Registration Certificate (the logbook) instead of the Tax Reminder, and the other items mentioned above.
You no longer need to fill any forms out at the Post Office.
why don't you do it online
its so easy,
provided you have the renewal notice.
I wouldn't do it any other way - unless it was a vehicle i had just purchased without valid tax
GSR600