Make sure you're in the adjust audio/video timeline at the bottom -- click on the icon to the left and above the timeline that looks like a clock face.
Next, put the playhead (the marker that shows where you are in the clip) wherever you want to trim the video. Next, hit command-T or select "Split Video Clip at Playhead" from the Edit menu. Then, you can delete the extra footage by clicking on the portion of the split track and pressing delete.
Hope that helps!