QuickTime opens as QuickTime Player.
Select File > New Movie Recording
The Video window opens with the view from the default video source. Check and change the audio and video preferences by selecting the Record Options. Unless there are external devices attached, choose the FaceTime Camera and Internal Microphone options.
Press the red Record button to begin the recording.
The button changes to a square Stop button. Press to stop.
The resulting video file can be saved as a .mov file.
Even if you click the red corner to close the window, QuickTime will prompt for you to save the file.
QuickTime allows for some in-app editing. The most simple editing is to Trim the video file (cut off some of the start and/or end of the file). Select Edit > Trim...
Move the yellow video frame handles so that they include the new beginning and end of your clip. Save the new edits.