Do not edit the Twenty Eleven theme. It is the default WordPress theme and having access to an unedited version of the theme is vital when dealing with a range of site issues. First create a child theme for your changes.
After you’ve done creating your child theme, find .entry meta in your CSS file, and add this:
display: none;
within the curly braces.
[sig moderated as per the Forum Rules]
Ok, I made a child theme. Where is this .entry meta?
Do I need to go into files ir just the editor in wordpress?
It is advisable to create child theme first as esmi said . To remove date from your wordpress posts all you have to do is to open the single.php file and search for the date function and delete it [Or comment this function]. You can follow the steps given at this link http://bornvirtual.com/wordpress/remove-date-from-wordpress-post/926/