To take a screenshot on a Pixel, make sure you're on the screen you want to capture. Then, locate the power button and the volume-down button - they're usually on the right side of the phone. Press them both at the same time, and you'll hear a camera shutter sound, indicating that the screenshot has been taken.
If you want to capture a long screenshot, like a whole webpage or a conversation, you can do that too! Just take a regular screenshot, then tap the capture more button at the top of the screen. Keep scrolling and tapping until you've got everything you need.