I don’t mean to undermine Ken’s solution, as it’s the best and most customizable.
However, if you care to take the beginners route, you can go to your admin panel Appearance -> Graphene Options -> Social Sharing Buttons.
Make sure the top two options are checked, and the third option has “Bottom of Posts” selected.
Lastly, simply insert your social sharing code. You can use a custom code, or you can get one at addthis.com or sharethis.com.