if you were approved for a credit card after you sent your credit card application, it doesn't mean you can use it right off. You should activate it, using the activation number your bank should provide in a letter, by phone or by Internet. You should also have the PIN of the credit card, which you can be given in a letter, or by phone, when you will activate your credit card.
Some people try to use the card without activating, as they already can give the card number where it is needed, but as there is no money available on the card, no payments can be processed. In some cases, there is a setup fee required to start using the credit card.
All credit cards terms, fees and rates mentioned in this article/post are actual on the posting date. See the current products’ Terms & Conditions on the issuing banks' websites.