Final answer:
The first step is to contact the bank or credit card company immediately to report the lost or stolen card or check.
Step-by-step explanation:
The first step to follow when a cardholder becomes aware of a lost or stolen card or check is to contact the bank or credit card company immediately to report the incident. This is important to prevent any unauthorized transactions and to protect the cardholder from financial loss. The bank or credit card company will then take necessary actions to cancel the card or check and issue a new one.