To add a place for an action (hook) you should use this function:

do_action( 'this_is_a_place_for_a_future_added_action' );

* it can be used anywhere in your theme

To add a callback for an existing place (hook), you should use:

add( 'action', 'action or hook', 'callback' );

such as in the following example:

function hook_test() {
$markup = '<div class="widget">';
$markup .= 'This is a hook !';
$markup .= '</div>';
return $markup;
}

or the same thing using a class:

class hooks {
public static function hook_test() {
$markup = '<div class="widget">';
$markup .= 'This is a hook !';
$markup .= '</div>';
return $markup;
}
}
add( 'action', 'before_widgets_right', array( 'hooks', 'hook_test' ) );

The example is on an existing hook in our default theme, to see all existing hooks in our default theme please click here