personal login – plugin

Hi all,

I was thinking to start with a personal project and I wanted to use wordpress as a “framework”.
One of the main things that I would like to add is my personal login form like a plug in. I have done it before with php and I have also worked with small OO projects in PHP but this is going to be a bit bigger.
I have some questions to how should I connect to the database and in general what should I learn to do something like this.
Well, for making my self more clear, how could I use the wordpress as a framework?
Is there a tutorial for something like this?
What should I study?

Kind regards,

Dinos