Adding Facebook Like button might be a very difficult task. It needs changes done on low level that can make your site stop working and bring a lot of troubles into your life.
In addition, to work properly, your site needs to support FaceBook Open Graph protocol.
With this plugin, all that you need is to install and activate the plugin. The plugin takes care of everything else automatically. If you want to change the default behavior,
you can use the plugin settings page "FB Like 4 Dummies, available under the Settings admin menu.
You can customize all available aspects of the Facebook Like Button Social Plugin: