To use ssh(1) I first start ssh-agent(1). I use -c to generate C-shell commands on stdout for csh-style shells and send that to eval.

eval `ssh-agent -c`

With ssh-agent running I use ssh-add(1) to add private key to it.

ssh-add ~/.ssh/<private_key>

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