bxwebber on "How to make jQuery function work?"

I desperately need help with making a small piece of jQuery code work.
This is the original script code:

$(document).scroll(function () {
    var x = $(this).scrollLeft();
    if (x > 100) {
        $('.topMenu').fadeIn(300);
    } else {
        $('.topMenu').fadeOut(300);
    }

});

I allready have a child theme in place. I've tried by creating a .js file (in a child theme js folder) plus a child theme functions.php, after reading several sources on this forum, but without succes. It's difficult for me to 'translate' given examples to my specific code. What should go where?

Thanks!!