Are you holding your phone portrait-wise (more tall than it is wide)? If so, that's your problem. Most camera apps expect the phone to be held landscape (more wide than it is tall) since that is the normal aspect ratio for video. Get used to holding the phone that way for videos. Besides, in my personal opinion, videos taken like that are extremely annoying to watch since you waste a ton of horizontal space watching it on a normal computer.
As for your existing videos, you will need to import them into a video editing program and rotate and re-render them.