For editing video, you can use inbuilt software such as Windows Movie Maker or Apple iMovie. There are also free programmes available online for editing video like YouTube Video Editor, Video Toolbox and We Video. Essentially, these video editors will let you add background tracks, transitions, subtitles and the like. You can experiment with a few of these and stick to one that works best for you. You can upgrade to something more sophisticated like Final Cut Pro or Premier Pro later. The length of the video depends on the nature of the subject. Some recommend keeping them short, to a couple of minutes, due to bandwidth limitations. For the same reason, make sure your file is not very heavy.