The best software to use is called Final Cut Pro. But for the sake of a short video to promote your project you can use anything. iMovie on any Mac computer is great and really easy to use.
Here's a link to some online video editors that are easy to pick up:
http://savedelete.com/10-best-and-free-online-video-editing-software.html
It's definitely not worth rushing this part of your project though as it's really important to have a good, smooth video which is clear and has some good narration.
