Using your camera, shoot your selected video. When finished, connect the camera to the computer using either a firewire cable or USB. Upload it to your video editing program or your computer.
Edit your video the way you want, keeping in mind where you're going to insert music for each part of the video. When you're ready to add music to certain sections (or the entire video), purchase and download the music file you want use.
Take the music file you've selected, and upload it to your video editing system. You can either use music from a pre-existing band or artist, or you can even use your own music you've created and recorded. Drag and drop the music file into the audio or soundtrack area of the editing bay, this area of the program is typically at the bottom of the editing bay.
Using the audio tools in your editing system, you can change the music with audio effects, including making it louder and softer and fade in/fade out. Most video editing programs should also have an audio mixer, which can change pitches, tones, volume and specific sounds on your audio track which can greatly change the mood and feel of your video. You can also download or purchase separate audio mixing programs to have a much more in-depth edit of the music you're selecting for your video.