Credit cards will most likely be the most common type of transaction you'll make.  To process a credit card transaction select "Credit Card" (F4) when finished scanning in all items of the transaction. 

A window will open prompting you to confirm the amount to be charged to the credit card.  If the amount is correct, you can proceed to click "Charge" (Enter) and the customer will then be able to slide their card.

When the "Charge" button turns grey and becomes non-clickable, the customer can slide their card to make their purchase.

If they have problems getting their card to process (for instance if the card's magnetic strip is too worn) then you can manually enter the card's information by clicking on the "Manual Entry" (ALT + M) button and a window will pop up with fields for all the information necessary to manually enter a credit card.

If a credit card is declined a line will be added to the transaction items displaying the credit card's last four digits, the error it encountered, and a price with the amount of "DECLINED."  This will most often occur when a customer doesn't have enough currency on their card to make the purchase.  This will appear on the customer's receipt should they still be able to pay through some other method.

If the customer cannot complete the transaction, you must delete the remaining items from the screen until only the credit card lines remain, and finish the "sale". The Declined line item(s) cannot be deleted - this is to ensure that the system records the attempted credit card purchase in the database, in case it needs to be referenced in the future.

See also:

Customer signature directly on the terminal:

How to process through a separate (non-integrated) terminal: