How do I edit a video to give a vertical video a horizontal aspect ratio?

I have a video that is taller than wide (vertical). Youtube rotated my video to be horizontal–it looks like all their video is horizontal. I think I need to put black bars on either side of my video to create the correct aspect ratio. How do I do this? I use a mac, I have quicktime pro and iMovie.
I have Apple iMovie and Quicktime Pro

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