Generate a key with:
ssh-keygen -t rsa
When you are entering the values for the key, remember to set the name and path appropriately.
Upload the public part to your server using:
ssh-copy-id -i ~/.ssh/root_key.pub email@example.com
Whatever you set the name and path to in the ssh-keygen part you will need to make sure it replaces the ~/.ssh/root_key bit in the second command.