#back-top {
           position: fixed;
           top: 30px;
           right: 10px;
           margin-left: 0px;
}
                     
#back-top a {
           width: 80px;
           padding-top: 10px;
           height: 24px;
           display: block;
           text-align: center;
           font: 11px/100% Arial, Helvetica, sans-serif;
           text-transform: uppercase;
           text-decoration: none;
           color: #999;
           border:1px solid #aaa;
           /* transition */
           -webkit-transition: 1s;
           -moz-transition: 1s;
           transition: 1s;
}

#back-top a:hover {
           color: #000;
           border:1px solid #aaa;
}

    <script type="text/javascript">
// спрятать #back-top в начале
           $("#back-top").hide();
                     
           // показать #back-top
           $(function () {
           $(window).scroll(function () {
           if ($(this).scrollTop() > 100) {
           $('#back-top').fadeIn();
           } else {
           $('#back-top').fadeOut();
           }
           });
                     
           // прокрутить плавно тело документа к 0px при нажатии
         $('#back-top a').click(function () {
         $('body,html').animate({
         scrollTop: 0
         }, 800);
         return false;
         }); $('#back-top').hide();
         });
</script>
<p id="back-top">
           <a href="#top"><img src="http://s21.ucoz.net/.s/img/wd/1/ar2.gif" /> Вверх <img src="http://s21.ucoz.net/.s/img/wd/1/ar1.gif" /></a>
</p>