WARNING: That's where the pleasure and pain of N900 starts. How much pleasure and/or pain depends highly on how familiar you are with linux. So make sure you know how to accomplish simple administrative tasks via a root shell without bricking your system and do regular backups.
Start "Application Manager" and install package "rootsh"
Open "X-Terminal" on your N900.
Gain root access:
Connect the n900 via usb to your pc and select "PC Suite Mode".
On your N900:
ifup usb0 ifconfig usb0 192.168.99.2
On your PC:
ifup usb0 ifconfig usb0 192.168.99.1
Please refer to http://wiki.maemo.org/N900_USB_networking for a complete guide.
Install OpenSSH client and server:
apt-get install openssh
Generate ssh key:
From this point it's more convenient to work remotely via ssh.
Copy the generated private key to your PC.
scp email@example.com:/root/.ssh/id_rsa ~/.ssh/id_rsa_n900
Put the public key to the right place:
ssh firstname.lastname@example.org "mv /root/.ssh/id_rsa /root/.ssh/authorized_keys"
Log on to your N900:
ssh -i ~/.ssh/id_rsa_n900 email@example.com
change /etc/sshd_config to denied password authentication:
# Change to no to disable tunnelled clear text passwords PasswordAuthentication no