Get the code
Create an account on addthis.com or sharethis.com, and look for a button that says 'Get the code'. You need to set some basic options so that the code will be generated to suit your needs. First, you need to choose the type of site - choose 'Button for website' or 'basic HTML'. Next you need to choose the style and format of your buttons.
With ShareThis you'll be given two pieces of code. The first one needs to be placed in the head section on the Site Settings page in the Pyplate admin area. The second one needs to be saved in the sharing template on the Comments and Sharing page. AddThis provide one piece of code which needs to be saved in the sharing template.
Individual social networks provide their own buttons:
Getting each button individually means you have more control over the way they are displayed.
If you're using page caching, you'll need to clear or rebuild the cache in order to see any changes on your site.