Wordpress - How to set up default values for a plugin?

You should do defaults at the time of pulling the data out. Never insert default values into the database. Defaults are default. Options in the DB override defaults.

How to do defaults for a serialized options array:

$defaults = array(
  'default1' => '1',
  'default2' => '2',
);
$options = wp_parse_args(get_option('plugin_options'), $defaults);