Integrate my own registration and login form with opencart

I have my own registration and login form that i need to use to register and login user to more that one website simultaneously. One of website is based on opencart.

I am new in opencart. So I need some guide/advise on procedure and strategy for integrate my registration and login form to opencart. The idea is when user signup using my form, the user data are in serted also in opencart user table. When user login using my login form, the user is also loged in open cart based website.

Thanks in advance