Help removing sidebars for mobile devices theme twenty twelve

I’m using a child theme with twenty twelve and need to make templates for phones and tablets with custom menus, footers, and no sidebars.

I’ve been searching for theme tutorials and how to use conditional tags for mobile devices but haven’t been able to get it to work yet. Can anyone recommend a good tutorial for creating your own mobile-ready theme?

My site is https://bayarearealestatetrends.com

Thanks!