It's from help.github.com.

Step 1: Check for SSH keys

$ cd ~/.ssh
$ ls -al
#Lists the files in your .ssh directory

Check the directory listing to see if you have files named wither id_rsa.pub or id_dsa.pub. If you don't have either of those files go to step 2. Otherwise. you can skip to step 3.

Step 2: generate a new SSH Key

To generate a new SSH key, copy and paste the text below, making sure to substitute in your email.
The default settings are preferred, so when you're asked to "enter a file in which to save the key, "" just press enter to continue.

$ ssh-keygen -t rsa -C "your_email@example.com"
# Creates a new ssh key, using the provided email as a label
Generating public/private rya key pair.
Enter file in which to save the key (/Users/you/.ssh/id_rsa): [Press enter]

Next, you'll be asked to enter a passphrase.

Enter passphrase (empty for no passphrase): [Type a passphrase]
Enter same passphrase again: [Type passphrase again]

Which should give you something like this:

Your identification has been saved in /Users/you/.ssh/id_rsa.
Your public key has been saved in /Users/you/.ssh/id_rsa.pub.
The key fingerprint is :
01:0f:f4:3b:ca:85:d6:17:a1:7d:f0:68:9d:f0:a2:db your_email@example.com

Then add your new key to the ssh-agent:

$ ssh-add ~/.ssh/id_rsa