New CMS area

Hi, I need to create a new area to be included in the flow with header, content, sidebar and footer. Such area (let’s call it “media”) should then be assigned to certain pages or categories. I’m working with a child theme from twentyeleven. I would like to know what files I must create and if I can get away with just adding a function or so… definetely I’m not going to get into editing main CMS files ^_^
Right now I’ve included the area in the content but it is not really efficient beacause as it changes in a page, I’ll have to change it in all the single articles that belong to that category page.
That’s why I want to be able to add it to the CMS and have it available with a similar call:
get_media();

thanx for any help that could put me on the right track

Alberto

(I’m not a php expert)