Once you have imported video into iMovie, it is helpful to title and organize the different clips. This will save hard drive space, keep your project more orderly, and will make it easier to find what you are looking for.
If you imported from the camera, you will often have long clips that contain several different scenes. Begin by splitting these large clips into several smaller ones, each containing just a single scene.
Move the playhead to the first frame of the new scene, and click Edit > Split Video Clip at Playhead, or, use the keyboard shortcut Apple + T. This will split the original clip into two separate scenes.