Final answer:
To manage the onset of a migraine headache, lying down in a dark, quiet room is an effective intervention. This approach minimizes environmental stimuli that can aggravate migraines.
Step-by-step explanation:
A nurse teaching a client who has a new diagnosis of migraine headaches should advise them to find ways to reduce pain at the onset of a migraine. The best intervention suggested is: "Lie down in a dark, quiet room." This method is an effective non-pharmacological option which can help reduce stimuli that might worsen the migraine, such as light and noise. In contrast, applying heat to the forehead, consuming caffeine-containing beverages, or engaging in strenuous physical activity may not be beneficial and could potentially exacerbate headache symptoms.