wordpress - how to add WYSIWYG to textarea with a specific class

[admin]

i need to add any wysiwyg to a textarea with a specific class in wordpress and i dont't know how to do it.

the best solution for me is gonna be to download some wp plugin where i should put a class name of textarea i need to add wysiwyg and plugin should should add any wysiwyg to it. is there such plugin? if there is not then how can i solve this problem?

also while googling i found this code:

wp_editor( $content, $editor_id, $settings );

but i cant realize where i should put this inside my code.

any help appriciated!