You can trim clips before you drag them to the timeline. Highlight your clip in the 'My media' section, then drag the in and out points at the bottom of the preview screen to mark where you want the clip to start and end. Once you’re happy, drag that clip onto the timeline.

To trim a clip in the timeline, either hover over either end of it until the arrows appear and drag in or out or move the playhead to where you want to split a clip and click the scissor icon.