Simple 'To Top' button script if browser goes below the fold.

1 - Script

$(window).scroll(function() {
	var window_height = $(window).height();
 
	if ( $(window).scrollTop() > window_height ) {
		$('.to-top').addClass('show');
	} else {
		$('.to-top').removeClass('show');
	}
});

2 - Mark Up

<div class="to-top">To Top</div>

3 - Style

.to-top {
  display: none;
  position: fixed;
  /* style/position where you like */
  bottom: 40px;
  left: 40px;
}

.show {
  display: block;
}