Apple provides comprehensive guides and "How Tos" for iMovie '11 on their website.
In addition, there is excellent built-in documentation in iMovie. When you open iMovie, simply choose iMovie Help from the Help menu.
Start with good quality video footage. One of the most important ingredients of a good video is to start with good quality video footage. If you shoot correctly, you'll get good quality footage. Avoid the mistakes of novice videographers. Check out "The Do's and Don't's of Shooting Video".