You have asked too many questions in one go. Follow the hyperlink and jump to page no. 6 for subjects' details.
http://www.fpsc.gov.pk/icms/admin/we...2017-Rules.pdf

One can only recommend you the optional subjects when you share your educational background. And as far as the registration is concerned then it starts in October of every year. You will get the details of how to submit an application on the FPSC official website.