The general concept of iMovie involves importing a video (called an "event) to your "event" library and then assembling pieces of an event or several events into the video you are creating, called your "project". Events can be both movies and still photos. You use your mouse to select pieces and then drag them to the "project" where further editing can take place. Simple editing can be accomplished merely by only selecting the sections of the video in the event library you wish to keep and leaving out the parts you don't need and dragging them to the project. More detailed editing can take place in the project. There are a few quick instructional sheets online:
The following short videos should help get you started with iMovie. iMovie has a huge variety of editing tools and capabilities, so these videos attempt to address a few commonly desired features. Browse through the menus and use the helpscreens if you don't see what you need here.