WordPress error: session_start() cannot send session cache limiter

Wordpress question: I get the session_start(); Cannot send session cache limiter – headers already sent (output started at /var/www/html/includes/myfile.php:1) in … error message when I activate my plugin. I had this on line 7: session_start(); My widget doesn’t work properly and as I was told it has something to do with cookies. I hope that you guys can tell me what is going on with my cookies. pls help me. Your error has nothing to do with cookies. “Headers already sent” refers to HTTP headers. The webserver cannot send any more http headers if the HTTP content has already started sending. Make sure that
is at the very top of the php script, and there are no BOM hidden chars or white space(s) in. Typical errors in plugins: 1. There is a whitespace char before the first PHP line
                    <- /n
// Your code
2. You unintentionally send whitespace char(s) to header (see line 12):
Plugin Name: MyPlugInError
Version: 1.0.0
Plugin URI: http://www.pluginerror.com/
Description: Check out the next lines to find out what the problem is here.
Author: Rookie error
Author URI: http://www.rookie_error.com/
// Now let's start our code
if (!isset($_SESSION)) {