First rule of shooting video on your smartphone. Hold your phone in the horizontal position. This way, if anybody is watching your video on any device other than a phone they won’t have to deal with annoying vertical sidebars. You want people to focus on the story, which is difficult to do when half the screen is filled up with a dark void of nothingness. Another plus of shooting horizontally is that you can use both hands to hold your phone, making for a less shaky shot.
If we haven’t convinced you to go horizontal, then this PSA from Glove and Boot should help.