WPyogi is right, of course. I can share the not as good of a system that has worked for me in the past as part of this whole learning process.
(use a text editor for all of this)
If you have only made changes to the style sheet then you need the bit they already posted and you need to save it as style.css and put it in a folder named twentyeleven-child. You can then highlight/copy the style.css that is active on your theme right now. You can save all of this in that folder on your desktop. If you have made changes to any of the other Twenty Eleven files you just copy and paste the whole thing, like if you made changes to page.php. Save those copy/paste files in the twentyeleven-child folder. If you have made any changes to FUNCTIONS.PHP, that’s different. You can’t cut and paste all of functions.php. You’ll need the opening tag <?php and then you can put just the changes in, if you aren’t sure what is and isn’t a change then copy and paste it into a text editor but DO NOT include it in the child theme. Next upload your child theme and then update twenty eleven and everything should be fine. If you made changes to functions.php upload, with the other files in your child theme folder (or afterwards), then upload a file called functions.php with just the opening tags (and closing tags…I can’t seem to find a consistent answer to this). Then you will have to take the copy you made of the original functions.php before you updated and compare it to the updated one and add just the changes to the uploaded blank one.