I suggest that you use a responsive width theme. Responsive width means the layout adapts depending on the size of the device being used to view your site. When responsive width themes are viewed on mobiles sidebars appear below the posts in order to provide as much space as possible for reading. See here for selecting one http://theme.wordpress.com/themes/features/responsive-width/
Go to Dashboard > Appearance > Themes
browse themes until you find one you like and click “activate”
or type in the name of the theme you want to use and
click the “activate” link when it appears
After you change your theme all you need to do is go here Dashboard > Appearance > Widgets > Inactive Widgets and re-install them where you want them to appear. The widget contents and settings will be the same as they were prior to changing themes.