If you SSH into the command line in your server, or SFTP into your server, or use github, you'll need a SSH key. On your Mac, type cd~/.ssh and you'll be taken to the hidden ssh directory. Hidden files receive a period in the front. A directory listing (ls) will give you the private and public keys, id_rsa and id_rsa.pub. If you already have these keys, you can write over them for the sake of this exercise. First copy both keys in backup files.
Then let's run the keygen command from the terminal: ssh-keygen -t rsa -C "email@example.com"
which will generate a new set of keys. One of the parameters "rsa" is a type of encryption scheme. ls -l will show the new and the old backup keys.
To add the new key to the ssh-agent: ssh-add id_rsa
Do a "cat id_rsa.pub" and copy the contents of your public key. Then go to your github account, and under Profile, find "Settings" and choose "SSH keys" from the left pane. Add "New Key" then test your new key by doing a pull to make sure everything still works with github. You'll also be copying the new public key over to your server.
To connect to github.com over SSH and test your new key:
$ssh -T firstname.lastname@example.org
If you receive a message telling you that you have successfully authenticated, then all is well.