First, regardless if you have one or multiple credit cards, the steps are the same, however you will go through the process each time for each Credit Card you are adding.
To add your first Credit Card, from the home screen, select "Chart of Accounts", which is typically found under the "Company" portion of your home screen. At the bottom of the screen, select the "Account" drop down, and choose "New". From here QuickBooks will ask you which kind of Account you are adding, and you will select "Credit Card".
The next screen will allow you to enter all the important information about your Credit Card such as the Credit Card Account Number, Account Number for your QuickBooks Chart of Accounts, and a description to tell it apart from other Credit Cards.
Next you will need to enter the opening balance for your Credit Card. If you just opened the line of credit, you may not have a balance, in this instance you can skip this step, otherwise choose "Enter Opening Balance". You will want to find your most recent statement from your Credit Card, and enter the Ending Date, and the Ending Balance for the card. Once this information is entered, choose save and close. You will then be taken back to your Chart of Accounts where you will see your new Credit Card with the total balance listed.
To run a report on the Credit Card, right click on the account from the Chart of Accounts menu and select "QuickReports"
To make payments for your Credit Card, you will need to enter the Credit Card Company as a Vendor in your QuickBooks Company file. As you are setting up the vendors information, the Account Settings tab will allow you to select the appropriate liability account to offset with this payment. In this case you would select the appropriate credit card that the payment is being made for. Once this is set you are ready to begin entering your bills as they are received from the Credit Card Company for Payment.