If you've already registered for Barclaycard online servicing, you'll just need the long card number and the card security code, on the back of the card. Then visit barclaycard.co.uk/activate
Register for Barclaycard online servicing. Then select the 'Register and activate' button, which'll take you through the steps to register and activate at the same time. To register, you'll need your credit limit, which you'll find on your welcome email."