First drag clips or portions of clips from Event to your Project. To delete unwanted frames, there are two ways.
1) Select the frames you want to delete so there is a yellow border around them. Then right-click/Delete Selection.
2) Select the frames in the clip that you want to keep. Then right-click/Trim To Selection.
For good Tutorials...
iMovie 08 Tutorials - mostly still relevant
iMovie 09 Tutorials - Mostly still relevant
Ken Stone's excellent iMovie 09 Tutorial
iMovie 11 Tutorials