<a id="go2top" class="go2top" href="#header"><span class="go2top-inner"></span></a>
<script type="text/javascript">
$(function(){
    $('#go2top').click(function(){
        $("html,body").animate({scrollTop:0},200);
        return false;
    });
    $(window).scroll(function(){
        var obj=$('#go2top');
        if(obj.offset().top>900){
            obj.show();
        }else{
            obj.hide();
        }
    });
});
</script>

.go2top {
    background: url("    bottom: 65px;
    color: #444444;
    display: none;
    height: 50px;
    margin-left: 510px;
    position: fixed;
    right: 160px;
    text-align: center;
    width: 50px;
}
.go2top:hover {
    background-position: -50px top;
}