In order to use DKIM, first thing you need is generate private and public keys.
There are many sites/tools you can use for this. But I have used the online DKIM key generating utility hosted EasyDARMAC .
On this site you need to specify your domain and the selector you use for DKIM. Selector is just identifier to identify DKIM record (specially if you have multiple domains and DKIMs). You can use any string as selector, but most of the time we use "mail".
When click on the Generate button, it generate, text for the DNS TXT entry, private key and public key.
DNS TXT entry will be like this:
Second part is private key. You shouldn't reveal this to anyone outside of your organization and must be kept as secret. Because if any malicious hacker get into this, that person can send email behalf of your domain.
You can save this private key string to .pem file and use it in your software to sign the email. We have used popular email software called Chilkat. They have comprehensive SDK to sign emails using DKIM private key. Check their DKIM reference here and you can also find example code here.
