http://wordpress.org/support/topic/move-main-sidebar-in-twen ty-twelve

Twenty Twelve, default theme in WP 3.5.
This code moves sidebar to the left.

Child theme CSS.
--

/*-- move sidebar to the left --*/
@media screen and (min-width: 600px) {
.site-content {
float: right;
}
.widget-area {
float: left;
}
}

/* for IE8 and IE7 ----------------*/
.ie .site-content {
float: right;
}
.ie .widget-area {
float: left;
}