MEMBERSHIP question

I'm trying to protect pages rather than posts. Most of the site is open to the public, but one section (Members Only) is meant to be protected. The Navigation has dropdown menus from which branch off other dropdowns, so call it a sub-sub-menu item. The sub-sub-menu items are pages and they are what I would like to protect. I am not a coder. Can someone please tell me step-by-step how to protect pages?

Also is there an authorize.net gateway planned?

Thank you.