[development] Style's stuff

Hello everyone: I need to change the look of certain pages, ie I have installed the organic group, and only I want to change the appearance of any content generated by this module with CSS. How can I differentiate them from other content? I know I have to change or rename the file page.tpl.php, but I do not know what I need to put as file name. I am a rookie in styles, forgive me. Thank you very much. I appreciate their support. Gustavo .-