Final answer:
To prepare an AED for use, turn it on first. The AED provides instructions and will guide subsequent steps, possibly including CPR or delivering a shock.
Step-by-step explanation:
When preparing an Automated External Defibrillator (AED) for use, the first thing to do is turn on the AED. This is crucial as the device provides verbal instructions and will guide the rescuer through the process. Once the AED is on, it will automatically diagnose the heart condition and prompt further action, which may include performing CPR or delivering a shock. AEDs are found in many public places and are designed to be used by anyone, making them a vital tool in cardiac emergencies.