There are tutorials online for help with iMovie (google: iMovie video tutorial).

Whether you learn from a tutorial, from your neighbor, or just by experimenting, please begin to learn these skills in iMovie HD:

• import a movie from PhotoBooth
• add photos by adding them to iPhoto first
• insert titles
• add transitions
• add video effects such as motion reverse
• cut a clip by highlighting it and using Command+T
• add voiceover