How to use a SSH key for GitHub on unix based systems (Linux or Mac)

Navigate to ~/.ssh

ssh-keygen -t rsa -b 4096 -C "your@email.com"

When asked to give your key a name I recommend naming it after the service you are using it for, so for me this will be github.

Create ~/.ssh/config with the following content

Host github.com
  HostName github.com
  User git
  IdentityFile ~/.ssh/github

Add the contents of the public key file (~/.ssh/github.pub) to your GitHub account https://github.com/settings/keys

Done! Now you don't have to specify which SSH key you want to use when using SSH to interface with GitHub.

This is how I do it.

Bonus: If you use multiple accounts you can do this in your config:

# account myfirstaccount (username)
Host github.com-myfirstaccount
  HostName github.com
  User git
  IdentityFile ~/.ssh/github-myfirstaccount

# account mysecondaccount
Host github.com-mysecondaccount
  HostName github.com
  User git
  IdentityFile ~/.ssh/github-mysecondaccount