custom authentication module

Hello,

I am going yo create a login module using my custom authentication module for member login. The scenario is as follows

1. User type in his info (username/password)
2. User is verified using an external api
   1. If user is authenticated then I get roles etc
   2. Otherwise throw the error
3. These users will not have access to the admin/edit cms area
4. For admin and all other users of the cms they will use the default login page

Can I get a head start into this? How should I do this? Do I have to create my own custom authentication provider? How do I check that cms users use the basic login functionality the other users uses the one I am going to create?