I am trying to find the type of authentication that I need for my web site

I want to make a login section in my web site like a form type with a username and password option that once a user logs in it will go look onto mysql

database to see the username and password are valid and point to a website that has another username and password login...

I am looking for a way to the customers to be able to login on my web site then go straight to the web site without login in to the site

Here is an example... customers usually logs into plesk by using their username and password....

I want customers to be able to log in into my web site and go straight into plesk without loggin in...// (is it called a portal login?)

I need a website that shows how to setup this authentication or a tutorial or someone that has the exact setup ..

I need also a copy of the html form ie action , post....

please I need help