S/MIME mail is built into Thunderbird, and probably Outlook.
PGP is available with add-ons, or in other mail tools, and has more history.
You can create sef-signed certificates quite easily and send S/MIME signed mail, then the recipient can send you back encrypted mail. That's quite secure, as long as you trust the recipient (you phoned them, you know them personally, you checked the certificate signature if paranoid - what are the chances of an imposter if you call and ask if they just got your email you sent 10 minutes ago?)
If you want hierarchical trust, you need a certificate authority to issue certificates to people - like a company HR department that has enough personal info to verify a person and issue an employee ID. Which the company can do itself, or buy a chaining SSL certificate from a regular authority like Thawte. But those won't issue certificates to individuals (at least, I don't think so) - you'd still need to trust the intermediate authority.
This is partly why you don't see encrypted email used so much - it's not hard, but no-one will step up and certify identities, unless some government agency does it and that's only for one country or state.
Computer people for years have used PGP with a crowd-sourced trust model, that's still used to sign security advisories etc. Technically you have to go meet people who are already in the trust pool and exchange enough government ID to get your PGP key signed.