The best way to add stylesheets to WordPress

To load scrpits only in front-end pages use:

 add_action( 'wp_enqueue_scripts', 'my_front_end_scripts' );
 function my_front_end_scripts(){
     wp_enqueue_script('script_handle');
     wp_enqueue_style('style_handle');
 }

To load scripts only in login use:

 add_action( 'login_enqueue_scripts', 'my_login_scripts');
 function my_login_scripts(){
     wp_enqueue_script('script_handle');
     wp_enqueue_style('style_handle');
 }

To load scripts only in admin use:

 add_action('admin_enqueue_scripts','my_admin_scripts');
 function my_admin_scripts(){
     wp_enqueue_script('script_handle');
     wp_enqueue_style('style_handle');
 }