This usually comes about because of missing, or extra characters within files. Generally this can be caused from problems during installation or upgrades on the WordPress software itself, or on a theme, or on a plugin.
The first thing to try in an attempt to fix this would be to rename your plugins folder. It can be found in the wp-content folder at the root of your WordPress installation.
The next thing to try is to re-upload all of the core WordPress files. Basically you would download, from wordpress.org, the same version of WordPress that your site is currently running. You then upload/replace all of your default WordPress files with the files from the backup. Do *not* replace the wp-content folder itself.
If the problem persists, you will also want to try changing your theme as shown here: http://wordpress.shadowlantern.com/2009/11/change-wordpress-theme-in-phpmyadmin/ … Or you can try re-installing your current theme by re-downloading the theme and re-uploading it again.
Note: If you already have the WordPress software or theme handy on your computer, do not simply re-upload them. Go and download it anew first, and then re-upload the new downloaded version. Some of the problems which can cause this error may have occurred when you first tried to download the files.
Another, increasingly less-common, problem is blank spaces before or after the opening <?php and closing ?> php tags. Check the files listed in the error you receive, and also check your index and wp-config.php files for these problems. Open the file in any text editor which does not insert secret characters (Do not use Microsoft Word as it throws in hidden characters you cannot see), notepad is a good option if you’re on a Windows Machine. Text Edit works well for a Mac. Remove all the extra blank spaces before or after those opening <?php and closing ?> php tags.