Noob question on how to encypt messages

I'm far from an expert myself, but I assume that you're going to use some kind of public key encryption, in which case you're going to need the public key/s of the person/s that you wish to send encrypted mail to.

Basically, you use their public key to encrypt a message (well, Thunderbird uses the key) and only they are able to decrypt it with their private key. You would do the same, giving all your friends/colleagues your public key which they use to encrypt messages to you.

I believe that Thunderbird does have built-in support for public key encryption but I've never used it and it sounds a little fiddly. The first thing you need to do is get a digital certificate from somewhere like Thawte.com (although I don't think they offer free digital certificates anymore). I'm sure somebody else more knowledgeable on the subject can fill in the blanks.

without any further addons, TB will support the S/MIME encryption standard. You have to get a certificate, there will be in fact a key pair created for you and the public key signed by the issuing authority. This can be obtained from globalsign, verisign or many other national or international agencies, some levels of certificates are free other you have to pay for. You can then import the keys/certificates into TB under options/advanced/certificates. For the exact use of the key pair, you should get instructions from the issuing agency.

Other way is to use pgp/gpg. For this you need addon called enigma.

https://portableapps.com/support/thunderbird_portable#encryption

But also here, you might try to read some texts in Internet on encryption etc. The enigma uses gpg for encryption and it will guide you in the process of crating your key pair and distributing the public key to your correspondence partners.

Once you have key pair, and you have public keys of your correspondence partners and they have your public key, then the rest might be adjusted pretty automatic.

Do you know what encryption method your correspondence partners use?
This is important for start. Ask how your partners encrypt messages and how you are supposed to do it. After that I can give you some more hints how to proceed.