If you're sending email via an email service then you'll need to decide what domain name to use as the token to attach to your email. This need not be the same as the domain name you use in the From address in your email, but does need to be a domain name you control and can create DNS entries for. Typically, if you own a single domain you would use that. If you own multiple domain names then you would use your “main” domain name, even if you use different domain names in the From address of the email you send.
Then you’ll need to ask your ESP what you should do next. They're likely to tell you to add a couple of records to your DNS, but the details will vary from ESP to ESP.
(If you're an ESP who supports DKIM Core and you'd like a link to your DKIM Core setup instructions here, contact us.)