If your movie has a black border it is because the resolution is lower than the project’s resolution. click on your movie in the time line and click on the movie preview, you should see gizmos appearing around your movie those will allow you to scale up your movie to make it fit the project’s resolution. Hold down SHIFT and drag one of the corner square, until it fits the movie project. if the movie doesn’t to fit in then it’s because your movie and the project’s aspect ratio are different. before you do any real editing try different project resolution settings until you see fit.
hope this help !