You'll need apache, and mysql installed. If they are not already installed the following command will install them.
$ sudo aptitude install apache2Run the following command to install roundcube
$ sudo aptitude install roundcube roundcube-mysqlConfigure roundcube
$ dpkg-reconfigure roundcube-coreNext create a symbolic link to apache's document root, in this case /var/www
$ sudo ln -s /usr/share/roundcube /var/www/roundcubeRestart Apache
$ sudo service apache2 restart