Umm, it is pretty simple.
There are 3 tracks, video, audio and title overlay. If you want to add music just drag and drop the audio file on to the audio track. Window movie maker doesn't separate the video and the audio of the original video, so if you want to edit out the audio from the original video you might need to do that with some other program. I recommend adobe premiere, but you have to buy it or you can dl it if you know where.
Just play around with it and you will get it. Movie maker is the basic of video editing.