Membership advice needed

I have designed a site where people will pay for access to the members area. I want to put the site on Clickbank so I will use their payment page

Here is basically what I want:
A sales page, from there they can go to the payment page.
Once they have paid they will then get a username and password and thy are directed to a sign up page (I want a custom sign up page not the wordpress default one. Then obviously when they sign up they can then go into the members area.

Can anyone suggest ways of achieving all this? I have already created the sales page and the members area but I don’t know how to make it private.
All plugins I have tried have only kept some content hidden but they can still get on the page. I want to keep it all completely blocked until they put in their password on the sign in page

Thanks in advance for any help