Turn on the histogram when setting the exposure manually because the phone screen doesn't accurately reflect what is captured on the SD card.
Exposure usually controlled by manipulation of ISO, aperture and shutter speed. The Spark has a fixed aperture so it's not adjustable. The ISO (sensor sensitivity to light) is best kept at 100 or 200 to avoid the grainy footage associated with higher settings.
The Spark's frame rate is also fixed - at 30fps. To get the subtle degree of motion blur associated with cinematic footage the shutter should be set manually to 1/60 sec (twice the fps). This slow speed usually lets in too much light (overexposed) which is corrected with a darkening neutral density (ND) filter. The histogram pattern is the key in choosing the right filter.
You can always choose auto exposure if all that is a hassle. But then the Spark will make exposure adjustments during your footage which seem unnatural to the viewer - you can edit those segments out later.
There are many many videos on the subject but they all relate those same basic principles.