Twenty Twelve theme: Right sidebar moves below content on iPad

This generally happens when something is too large and breaks the box model, so that’s where I would start.

i did fix the width on my page – but i would like to keep it that way ??

The twenty twelve theme is responsive. It uses media queries for each screen size.

If you want to change the appearance you will need to create a child theme and overwrite the media query for that screen size.

I did create a child-theme – can you guide me through that other stuff?

responsive design are used to prevent horizontal scrolling in your theme. If you look at your style.css file you will see rules that start with

@media screen and (min-width: 600px).

In this statement if the screen size drops below 600px the rule would not apply.

You can also do a range with a rule like
@media screen and (min-width:600px) and (max-width:1200px)

This rule would only apply to screens with a with greater than 600px and less then 1200px

You would have to go through your style.css style sheet and find the rules that apply to the screen you want to change.

If you want to see the media query in action you can open the site on a large screen and narrow it down.