What blog platform are you using? If you're using Blogger, then you should read this:

To add social bookmark buttons to the end of every post you can use a service called addthis.com. In order to add the buttons you will need to modify your template.

First make a backup of your template before modifying the HTML by clicking on “Download full template.”

Go to Template Edit HTML and check the “expand widget templates” option.

Then Find the line <div class='post-footer'> and paste the code right after that line.