We use ubuntu servers because they are are awesome to work with. There are however some technical things that you may run into when installing a WordPress CMS on an Ubuntu server.

Enter FTP Credentials To Install Plugins

You may be confronted with "Please enter FTP credentials..." when trying to install a plugin. This is because the user for the server is not set up properly in the WordPress CMS. Solution coming at the bottom of this post.

Cannot Create a Folder in Uploads

Another problem that you may be having is that you cannot upload pictures. You will get an error that says the software had a problem creating a folder.

Update and Install Themes Problem

This is similar to the plugin issue. The WordPress scripts do not have the necessary permissions to install or update files.

The Solution to the Above Problems

You have to change  chown in the terminal line the following. This will give your WordPress scripts the ability to make the necessary changes. You will be able to upload and update with no problems after this. Log in as "root" and enter the following command in the terminal line.

chown -R www-data:www-data /var/www 

If you are still having problems

If you are still having problems then you can try setting FTP credentials in your wp-config.php file.

define('FTP_HOST', 'server ip address');
define('FTP_USER', 'www-data');
define('FTP_PASS', 'password');

Take the next step as a LAST resort.

If you are still having problems use this as a last resort. Put this in your wp-config.php file as well. It will change permissions to your directories in your wordpress CMS.

define( 'FS_METHOD', 'direct' );
define( 'FS_CHMOD_DIR', 0777 );
define( 'FS_CHMOD_FILE', 0777 );

I hope this was able to help you. :-)