HTML help...I created a HTML file with code an all with a sample header, and a CSS file with one style change; to change the header color?

Well, there's either something wrong with the HTML or the CSS. You don't show us either, so it's hard to say.

On some web servers, the file names are case sensitive. So if you have Stylesheet.css as a file, and your HTML links to stylesheet.css, it won't find it. That's one thing to check.

Try this:

HTML:

<header>Hello, I'm a header</header>

CSS:

header {

background-color: green

}