How to upload a video from my iPhone to Youtube without it being narrow?

I just uploaded my first youtube video, but it shows up too narrow. How do I upload a video from my iphone and have it shown as the width of the screen on YouTube?

If the iPhone was in portrait mode the video is going to be narrow. When it is uploaded there will be a black border surrounding it. There's no good way to fix this, if you were to stretch it the video would become distorted. You can however prevent this in the future, just make sure you are holding the device in landscape mode (on its side). You may need to turn portrait orientation lock off by swiping up. Hope this helps!