You'll need apache, and mysql installed. If they are not already installed the following command will install them.
sudo aptitude install apache2
Run the following command to install roundcube
sudo aptitude install roundcube roundcube-mysql
Next create a symbolic link to apache's document root, in this case /var/www
sudo ln -s /usr/share/roundcube /var/www/html/roundcube
sudo service apache2 restart
Now browse to your machine at http://myhostname.mydomainname.com/roundcube