Position the playhead where you want it to start, then drag the clip's left end to the right; it will automatically stop where you put the playhead. Then do the same thing to mark the clip's end. Finally, share it. Hit the Export button, then scroll down to the Video parameters. To shrink for sharing, choose H.264 and change Quality according to LR's guidelines. If the clip is headed, say, for Facebook, choose Medium. Note: If you use Publish Services to share, you'll need to adjust the quality settings before upload. If a clip is too big, Facebook will reject it.