How to create an upload file in html/css/js

I've been working on a website for about a month, but I've hit a roadblock on one feature I would like to add. I know how to create an upload file button but I want to create a system so that I can receive the file they upload to me. What would be the easiest way to create this function? I would like to create the feature in the website rather than using an external program. Any help is appreciated, Thanks!