Upload your video, so someone can take a look. Here's how... http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/
Two things to play with...
1. Right-click the file in the bin (top-left of the window) and click ASPECT RATIO. Play around with the many settings options.
2. With the clip on the timeline click its FX button and then click the small green + item on the toolbar, second from the left.
In the preview window, use the handles (dots) to draw a mask that's the shape desired. UNtic the "mask inside" box.
Click the large green + button on the toolbar (far left) and click the TRANSPARENCY item. Drag the slider to the far left. Export.
To achieve the effect desired you may have to utilized the SCALE or ZOOM effects.