encrypt emails

I have gotten some excellent answers from Linc Davis about encrypting emails. However, I can't find his last post. I want to make sure I understand how

encrypting emails works. The certificate from Comodo links a person email address with both a private and public key. After you add this certificate with both private and public keys you request the public key from the person you want to send an email to. Then, I am not sure what you do with that person's public key? Do you add it to the keychain on your computer? If so, How do you add someone's public key to keychain?

If the next step is to add this person's public key to your keychain, then do you use Apple mail to send them an email and the encryption icon will be highlighted? Also, are any documents which I attach to an encrypted email also encrypted? I also wonder if the people I got the certificate from can decrypt an encrypted email?

Any answers to this questions are much appreciated.

Thanks,

Elaine