There are few possible reason you are getting this warnings:
1. There is some space before your first <?php tag, sometimes, your editor can add some garbage value. Check it has not added anything like that.
2. Move you session_start(); to the top, exactly after <?php, sometimes a white space before the session_start(); could mean, you are printing something, before the session has started.
3. If the session has already started then do not call session_start() again. Like below,
if(!isset($_SESSION)) session_start();
Any of the three could solve the problem. Let me know, if it does solve it.