There are several ways to control access to your WordPress website so that you can let customers get your information instantly online, and protect it from non-customers at the same time.
By far the easiest tool is Wishlist Member.
This software is a plugin that lets you protect content by post or by category so non-members can't get it.
But customers, as soon as they complete a purchase, register and get access instantly.
This is more than just controlling who registers at your WordPress blogsite.
It's a controlled access to membership "levels," and a customer can have access to one or more levels depending on what they've bought.
This is the software that we use, and that several of our clients have used with great success.
Learn more at the Wishlist Member website.