How can I install modules using the FTP method? I got a error when trying to install modules. What should I set up for the FTP? I just learned Drupal. Who knows what happen? I am using Drupal 7.7.
As kalabro said: 

Make sure the folder /sites/default is OWNED by the user that executes the drupal scripts. 

On most Ubuntu installations, this is the user Apache user www-data.

chown www-data sites/default