How do I encrypt email on MacMail?

Hello, I am trying to send some sensitive information over email using MacMail. The company is legit. I began the process and they responded with the info. I needed to send. How do I encrypt the material to get it to the intended company safely? They say their email server is secure, but I don't know how secure mine is. Also, would it be more secure to send the email using MacMail through iCloud.com?

I tried to use the certificate process, but I don't think I can request a certificate as a private individual, and if I can, I have no idea where to send the request when prompted by the certificate help "wizard."

Thanks for any and all help.