Sometimes you make a mistake and click the record button on your mobile device and while shooting that video you turn the camera to a different orientation. This will show you how to rotate it 90 degrees.
Open iMovie and import your video
Drag your movie into the timeline
Cut out the parts of the video you don't need.
Select the movie in your timeline
Choose the "Cropping" tool in the proprieties window
Choose "Crop to Fill"
Choose the "Rotate tool" in the direction you need
You'll want to "Share video" to make it its own MP4.