You would have to navigate to the WordPress Twenty Eleven Theme folder in your WordPress installation at the following location:
/wp-content/themes/twentyeleven
Next open up the file called header.php
and find the part that says <h1 id="site-title">
. Finally, add your logo image before this along with anything else you may want.
Bare in mind you may need to use some CSS, like display: inline;
for example to get it to sit on the same line as the text.