Adding CSS to a shortcode plugin

I’m working on developing some plugins for a wordpress site. and I would like to have my css code stored in a separate css file in the plugin folder. what do i need to add to tell it to look for css style info in my css file (say mystyle.css” ? My current code is;

//define plugin defaults
DEFINE("DDCOLLAPSABLE_HEADER", "Header Title");

//tell wordpress to register the dd_collapsable shortcode
add_shortcode("dd_collapsable", "ddcollapsable_handler");

function ddcollapsable_handler($incomingfrompost) {
  $incomingfrompost=shortcode_atts(array(
    "headertitle" => DDCOLLAPSABLE_HEADER
  ), $incomingfrompost);

  //run function that actually does the work of the plugin
  $ddcollapsable_handler_output = ddcollapsable_function($incomingfrompost);
  //send back text to replace shortcode in post
  return $ddcollapsable_handler_output;
}

function ddcollapsable_function($incomingfromhandler) {
  //process plugin
  $ddcollapsable_output = "<div class='box effect8'>";
  $ddcollapsable_output .= wp_specialchars_decode($incomingfromhandler["headertitle"]);
  $ddcollapsable_output .= "<br>";
  $ddcollapsable_output .= "Hello World!";
  $ddcollapsable_output .= "</div>";
  //send back text to calling function
  return $ddcollapsable_output;
}
?>