Follow the GitHub guide. When you screw up tons of people are familiar.
Apple stopped saving SSH passphrases in its keychain in macOS 10.12.2, which is a pain in the ass because Git prompts for your passphrase after every command.
Long term fix
Create/edit this file to fix it:
Host * UseKeychain yes AddKeysToAgent yes IdentityFile ~/.ssh/id_rsa
Host entries are matched first ➔ last, so this may need to be repeated in each additional host entry.
Short term fix
Needs to be repeated after every restart.
ssh-add -K ~/.ssh/id_rsa
You could add this to your bash profile so every terminal session starts, but people say it’s slow.