CSS Twentyeleven / Sidebar has to be changed IN the CSS Stylesheet. Sidebar on the left side

Try to edit your style.css in the following way:

Find this: #secondary

And edit it in this way:

#secondary {
    float: left;
    margin: 28px;
    width: 22.8%;
}

And then find this: #primary

And edit it in this way:

#primary {
    float: right;
    margin: 0 -55px;
    width: 80%;
}

By doing this, your sidebar will move to the right. You can keep working on these css properties till you get your perfect layout.

Note that this works for TwentyEleven theme.