To use ssh(1) we first need to start ssh-agent(1). We use -c to generate C-shell commands on stdout for csh-style shells and -s for Bourne-shell.

eval `ssh-agent -c`

With ssh-agent running we need to ssh-add(1) our private key that we want to use.

ssh-add ~/.ssh/<your_private_key>

That's it, ssh-agent will now hold that private key and use it for public key authentication. You can list all identities currently represented by the agent with the ssh-agent -l command.

To set this up automatically on login, just add these commands to your shell configuration file. As an example, for tcsh(1) use ~/.tcshrc.