You can easily cut out unwanted footage and keep only the best parts for your movie. Use the iMovie HD controls to select exactly the frames you want to keep or get rid of.

You can also use "direct trimming" to trim your clips by dragging the end of a clip to shorten or lengthen it in the timeline viewer.

And don't worry about changing a clip, iMovie always keeps a copy of the original, so that if you shorten a clip too much, you can always undo your changes one by one or restore the clip to its original state. You can even retrieve a deleted clip from the iMovie Trash after you delete one.