Connect to SSH DigitalOcean , you can use these tuts with other instances as the commands are the same with most Linux distributions.
Initial Server Setup for Ubuntu.
How to set up SSH Keys
How to create public/private keys using Putty. If your using a Mac you can use OpenSSH.
This information should get you in the right direction. I don't know of any 3rd party security plugins that I would trust, also I want to correct myself from the above comment, not an SSH login, but a SSH key Authentication with a passphrase.