#back-top {
position: fixed;
bottom: 30px;
margin-left: -150px;
}

#back-top a {
width: 108px;
display: block;
text-align: center;
font: 11px/100% Arial, Helvetica, sans-serif;
text-transform: uppercase;
text-decoration: none;
color: #bbb;

/* transition */
-webkit-transition: 1s;
-moz-transition: 1s;
transition: 1s;
}
#back-top a:hover {
color: #000;
}

/* arrow icon (span tag) */
#back-top span {
width: 108px;
height: 108px;
display: block;
margin-bottom: 7px;
background: #ddd url(up-arrow.png) no-repeat center center;

/* rounded corners */
-webkit-border-radius: 15px;
-moz-border-radius: 15px;
border-radius: 15px;

/* transition */
-webkit-transition: 1s;
-moz-transition: 1s;
transition: 1s;
}
#back-top a:hover span {
background-color: #777;
}

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.3/jquery.min.js"></script>

<script>
$(document).ready(function(){

// hide #back-top first
$("#back-top").hide();

// fade in #back-top
$(function () {
$(window).scroll(function () {
if ($(this).scrollTop() > 100) {
$('#back-top').fadeIn();
} else {
$('#back-top').fadeOut();
}
});

// scroll body to 0px on click
$('#back-top a').click(function () {
$('body,html').animate({
scrollTop: 0
}, 800);
return false;
});
});

});
</script>