modifying plugins layout

Hello guys, I want to ask you a general question, I would like to actually modify how some plugins are displayed on the web page of my blog, but the way I want them to show is not present on the plugins options, so I need to hack into them, the thing is I don’t know where to look, can someone help me? maybe telling me where to read on the codex and common places I have to look about the layout of the plugins, thanks!