Final answer:
The first step after receiving an AED at a cardiac emergency scene is to turn it on so that it can provide verbal instructions and analyze the patient's heart rhythm.
Step-by-step explanation:
The first step to perform once the Automated External Defibrillator (AED) arrives on the scene is to turn on the AED. This is critical because the device will provide verbal instructions for its use and will automatically diagnose the patient's heart rhythm, advising whether a shock is needed. The AED's instructions must be followed precisely, and if CPR is in progress, it should be continued until the AED is turned on and ready for use. Once the AED is on, follow its prompts which may include attaching the pads to the patient's chest.