  <?php echo get_stylesheet_directory_uri(); ?>/css<br>
    <?php echo get_template_directory_uri()?><br>
    <?php echo get_stylesheet_directory()?><br>
    <?php echo bloginfo('stylesheet_directory'); ?><br>
    <?php echo plugin_dir_url();?>

To find the plugin or theme directories see codex, require vs include. These return the following

http://example.dev/wp-content/themes/basictheme/css

http://example.dev/wp-content/themes/basictheme

/Users/arielguzman/mampsites/1stBootstrapToWP/wp-content/themes/basictheme

http://example.dev/wp-content/themes/basictheme

http://example.dev/wp-content/plugins/

Functions.php

Sitepoint --List of customizing functions for admin

Customization API

Codex