Creating a domain registration system?


i want to know how to create a domain registration system in PHP, not specific details, just an overview of where do i start and what do i do? do i need to communicate with an API? how do i transfer payments? and other stuff, just some basic pointers in the right direction will be alot helpful, my client has already become a registrar so i just need to do the technical system.

Any help ANY HELP will be appreciated.