You Have a Flashlight
Day can quickly turn to night if you're adventuring in the wasteland. And sometimes, that darkness can make look and enemies difficult to see. And that's why you'll want to flip on that trusty flashlight you didn't know you had.
Your Pip Boy (not the real-life Pip Boy) comes with a flashlight that produces a greenish hue. The light doesn't go very far but it's going to be essential when you get stuck in a place with less-than-optimal lighting. To turn it on hold down B (Xbox One), Circle (PS4) or Tab (PC).
There's another way to light up your surroundings. If you find the right materials, you can construct a headlamp for your Power Armor. Go to a Power Armor station to build it.