First create your key (something like this but google for a better scope)
ssh-keygen -t rsa -C "email@example.com"
Remember you only need to do this once
then on each Ubuntu server you want to log into. Changing username to your username and xx.xx.xx.xx to the ip or fqdn of the server.
cat ~/.ssh/id_rsa.pub | ssh firstname.lastname@example.org "mkdir -p ~/.ssh; cat >> ~/.ssh/authorized_keys"