How can I remove these black borders from the left and right side of my video?
How exactly you do this will depend on the video editing software you have available, but basically you're going to create a new project with an HD frame, then import the video and zoom into the video until it fills the frame. The action stays enough within the center of the frame that (I think) you should be able to do this without a problem. You will lose quality this way, but there's no way to avoid that.If you have After Effects or Premiere Pro available, you might want to try using one or the other of those so you can attempt to run stabilization (either warp or the older tracking version) on it as well.I would also second what Mike says: in the future, hold your phone sideways to avoid shooting in "tallscreen".