A Public Key Certificate lets you sign and encrypt you email. Actually - to encrypt your email - the person you are writing to needs a certificate.
Most certificate services require that you provide a degree of identification so that they can assert that in some sense the person signing the email is indeed the person they say they are.
If you only want to correspond privately with someone you know - then there is no necessity for any formal identification process in order to obtain a certificate.
You can go to the application form and fill out the "Request a Certificate" form, then email me to remind me to authorise your certificate request. Come back to the application form in a day or so and go to the "Check on a Pending Certificate" page. If it has been authorised - you should get an "Install Certificate" button.
Once the certificate is installed - you need to send a Digitally Signed email to your friend. You might have to set some options in your email client to be able to do this. He or she must then save the certificate (the public key) in his/her address book.
When they next send an email to you - they will be able to encrypt it with your public key - so only you can read it. Again - they might have to fiddle with some options.
You might suggest to your friends that they too should get a free certificate so that you can send them encrypted email.
Hope you enjoy using your free certificate.