Sound is one of the most difficult aspects of video production. If you can it’s best to use a separate microphone plugged into your camera when recording speech so you can get it closer to the person speaking. Clip-on microphones can be attached to clothing.
If you have to use the camera’s built in microphone it’s best to get your camera close to the person speaking or the source of the sound.
Do a test of the sound and the sound levels before you start filming.
When thinking of the sound for your video remember that there are many copyright issues with using live and recorded music to accompany your film.