Say, I have my site from a host provider, during the setup of a certificate, it says that I need to fill the blank of domain name, first name, etc. What is "mandatory" to fill? If I do add personal info to the certificate, e.g. my site does not have a personal info, when filling those info when setting up a certificate, will my personal info be shown from the certificate or will it only show that the certificate is valid or verified with no personal info?
No. They won't show it up for normal certificates. There are many types of certificates. Most used and cheap are domain validation ones. The data that they are getting from you is to identify the owner. It is for their internal use. And your info won't be shown unless you use an Extended validation certificate. (The one with a big green bar which holds the name of the owner). It costs more than. Usual ones.