Before you get too far down this road, let me caution you that it's impossible to reliably secure e-mail being sent through a webmail system such as Yahoo, Gmail, Outlook.com, etc. unless you are encrypting the message using some other tool on your local system and then only copy and pasting in the resulting ciphertext. Any other method results in your encryption "private key" being stored on a system outside of your control and thus renders the encryption suspect to useless.
You want to look into a standard e-mail client (I personally recommend Mozilla Thunderbird) and use either PGP or SMIME to encrypt the mail. You also need to use a provider that gives you access to your mail via POP3 or IMAP (Gmail and Outlook.com do, Yahoo does not). The downside to the former is that it can be esoteric to use with the chain-of-trust concept and requires multiple programs to interoperate (e.g. Thunderbird + GnuPG + Enigmail). The downside to the latter is that to use "trusted" certificates, you have to purchase the certificate for a non-trivial amount of money.