Getting the sidebar on every page / post?

Hi, I’m new to self hosting. Had a WP.com blog for 4 years but am on a sharp learning curve regarding css and customisation. Is there anyone who can help, please…?

I’m using theme TwentyEleven.
I found the stylesheet under Appearance: Editor.
There, I found the section on structure.
Within that I found this reference to the sidebar:

/* Right Content */
.left-sidebar #primary {
float: right;
margin: 0 0 0 -26.4%;
width: 100%;
}
.left-sidebar #content {
margin: 0 7.6% 0 34%;
width: 58.4%;
}
.left-sidebar #secondary {
float: left;
margin-left: 7.6%;
margin-right: 0;
width: 18.8%;
}

At present the sidebar is only showing on the Home Page.
What do I need to do to get it to show up on every single page / archive / etc? Do I input some additional code in that little chunk of html I pasted above?

Thank you!