merging of plain php sites admin panel and wp’s admin panel

Hi friends,
I am new user of wordpress .
I have integrate wordpress with my site(it is made in plain PHP)
I installed successfully but now I need a single login screen and after logged in, users will redirect to a common Control Panel. Presently, I have two control panel one is my site(which is created into plain php) another one is “wp”(after wp installation.)
2. Need to register users or login admin panel (control panel) from the non-wordpress based site. he/she goes to normal site control panel then when he click on wp-admin link he goes to wp-admin panel with out login to wp-admin panel But I don’t how can I achieve the following:

[A] On registration time, store password in the user table.

[B] How can compare the password with database password on login time?

3. Where can I find out WordPress Function References?

I hope that, the comment is active and will help me as soon as possible

Pls Help Me