The way you are phrasing this question makes me think that you are in way over your head.

You need to implement a device with a USB Video Device that takes your source signal as HDMI, VGA, Composite or whatever you have.

Your best bet is a complete device like this.

If you really want to make something yourself you can have a look at the cypress app note AN75779 to get a rough idea.

Here is another article about it.

Another one.

And here is a deprecated Microchip part.