Try to avoid having a light source / daylit window directly behind you.
Do however make sure there are some lights on in the room. Webcams often perform poorly in low light.
Test your setup
Windows
Open the Camera ‘app’ (Start Menu > type 'Camera') to get a preview of how everything looks.
MacOS
Open an app that can use the camera, such as Photo Booth, FaceTime, or Messages. A green light beside the camera glows to indicate that the camera is on and you can preview how everything looks.