You will need to generate a pair of keys on your local machine and then copy the public key to the remote machine.

Look for instructions that are specific to whatever server that you're using. E.g., Github, Site5.

Basically, you want to check if there are existing keys:

ls -al ~/.ssh

If you don't have keys there, you will need to generate some.

A problem that I was running into was that Ubuntu would keep prompting me to enter my key passphrase every time, which defeated one of the reasons for passwordless login.

The messages looked like:

Enter passphrase for key '/home/username/.ssh/id_dsa':

To get rid of the password prompt on each login, type ssh-add and enter your passwords for the last time.