This plugin will add a "back to top/ scroll to top" icon automatically to your blog.
Make sure you have the following code in your theme's footer.php :
<?php wp_footer(); ?>
If not, go to Design >> Theme Editor >> Select "footer.php" from right hand list >> and paste
<?php wp_footer(); ?> just before the '' tag.
Do it via my blog - http://htmlblog.net/ - enjoy!