Template still remain 100% width

Template still remain 100% width in every template I choose. Im creating my new template but i see in the blog page the sidebar appears below article and not to the right.
How can I solve this?

Your question is one of design, and the answer is going to be very specific to the theme you are using. So in order to be helped, you will need to let us know what theme it is.

Also i’m not 100% sure what you asking, ‘template’ in Grav means the Twig file that is used to render output, although I feel like you are asking about a ‘theme’ in general. The sidebar appearing below might be related to lack of support of flex (again depends on the theme), or could be simply to do with your device/browser. Many themes, will wrap sidebar underneath in mobile views when space is more limited.

I wanna learn about themes (i found a useful site where they explain it as well). Doing this I will get more control about themes creation and layout in grav, this is very important.
However, im talking about Antimatter theme…

Nothing to do!! I tried another good theme (Afterburner2) and in my site the blog post remain fully width and all other plugins are moved below… how can I customize the skeleton of my site?

You can modify the theme under your users folder. Just search for the template you want to change and do your adjustments in the certain files (CSS, JS, or Twig). I think for starting it would be usefull to take a look at the GRAV documentation which is really good structured (https://learn.getgrav.org/themes). I dont know which technical background you have but even if you’re not very familiar with Twig etc. with a little time and the docu you might get your wanted customizations done.

Most themes have a responsive layout and push below the sidebar when the width is not enough. Post a screenshot of what you see.