Place your timeline playback (orange) marker at the start of your film (click "Home" on your keyboard) and then click the "{ " symbol at the bottom of the preview window. This sets the start (range) or in point. Then move the orange marker to the end last clip ( "End" key) or anywhere before and click the "}" symbol and this will set the End or end range out point. A blue range indicator will appear across the top of the time line whose ends can be dragged with the mouse if you only want to export part of your movie.