[Twenty Twelve] How to add widget to post ?

start with creating a child theme https://codex.www.ads-software.com/Child_Themes

then possibly use a plugin such as https://www.ads-software.com/extend/plugins/twenty-eleven-theme-extensions/

So i don’t have to mess with the child theme if i use that plugin right ?

if the only changes are made through the theme options or plugins, you can work without child theme.

just be aware that any edits to the theme files will be overwritten with the next upgrade of the theme.