1 zoom in to where you want the clip to start,
2 press the "M" key on your keyboard and a marker will appear there
3 zoom in on where you want your clip to end
4 press the "M" key on your keyboard again and you will have a second marker..
5 make sure snapping is enabled
6 highlight the area between the markers, with snapping it will automatically snap to the marker and highlight between them.
easy as that O.o
as for "Masking" that can be found in the pan/crop section of the program which is pointed out in this screencap:
then you follow these steps:
and use the keyframe timeline to set changes in the mask (depending on how fancy you want this to get you may have to use more than in the image)
and if you want the part of the screen to be blank (or a hole instead of a solid shape) choose the negative option:
~~~~~~~~~~~
I've had the screen name for a very long time @_@


