following a video tutorial on how to chroma key
All you need to do is put a white solid below your track or set your Composition background color to white.
Keylight creates an alpha channel which means everything you see that is black is actually transparent. Putting a white solid or any other footage below your keyed footage will show you that.
Either the tutorial you are following is a very bad one or you missed something.
Actually I found a video on I think video co pilot and they said to press the white checkered background before the comp..the other tutorials didn't say that. Nothing about a white solid. Think it's solved now though