YouTube does not have a webcam feature to record videos. You can use software on your computer to record and save a video. Then, you can upload the video to YouTube.
Popular software includes Photo Booth for Mac OS X and Camera for Windows 8+. If you have another operating system, you may need to look up how to record videos using your webcam. You can usually find info in your device manual or online.
If you have a mobile device, you can record videos with your phone's camera and upload them with the YouTube app.