How to use a video once it's uploaded?

So I have some videos on my phone that I need to upload onto Amazon in reviews. I've gotten the videos from my iPhone to my iMac desktop, however the computer won't let me choose the video to upload into the review box. Am I supposed to reformat the video or something? I absolutely have no clue. I really need to be able to post these videos where I need them but I've not been able to. Thanks!