Hi,
The iMovie screen has a 16:9 aspect ratio that cannot be changed. In other words, it is widescreen. When you take a video with your iPhone held vertically you will get a 9:16 aspect ratio that is vertical instead of wide. In order for the video to fit iMovie's 16:9 aspect ratio iMovie will place black bars on each side of the video image. You can click on the crop button in the tool bar at the upper right of your screen (the one that looks like intersecting angles), and chose the Crop to Fill option, and adjust the aspect ratio by cropping with the adjustment rectangles in the preview screen.
But then your video will look highly cropped, like this:
In the future, be sure to hold your iPhone horizontally when you shoot, if you intend to edit your video in iMovie.
-- Rich