100% wide slider below menu (with sidebar)

Hello,

First post and everything, wow. Well, I have a question to all of you brilliant people: how do I add a slider below my menu?

TL;DR?
How can I add a slider of 100% width in between the header and the main content, when I also have a sidebar, such that the slider comes above the sidebar and content?
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I want to add a slider of 100% width, that appears right above the main content area and sidebar. I’m thinking this would be part of the header then (unless, of course, I add another div in between), but I’m not quite sure how I would do this, any ideas?

To clarify my layout, I want:
1. A header image at the very top
2. The navigation right beneath that
3. A full-width slider beneath that
4. The standard layout with a sidebar and main content area.

The theme I am modifying is TwentyTwelve. I consider myself to be very good at CSS and HTML, fairly good at JS, and completely fine with PHP. However, I am a complete beginner as far as WordPress goes, and so I would appreciate it if you would take an extra minute to explain to me what I would need to add to, for example, enable for a slider, and how I would add it through the CMS would also be appreciated.

So far, the only things I’ve been able to think of is either changing the header-file to add for another image (possibly by adding a div after the nav), or adding a div right after the header (in the main template) with 100% width that would accept a slider or image. However, I am not quite sure of how I would do any of this, so all help is appreciated, even if you don’t have an exact solution to my problem!

Kind Regards
Leo