Creating a user registration and authentication system is no small thing. It is also probably well beyond the scope of this forum, due to space limitations if nothing else (800 words maximum per post).
Have a look at the excellent PHP Freaks site click here who have fully tutorials on user authentication, among other things.
You could also try Hotscripts.com for some examples of ready made scripts.
Same or similar table structure is academic really. Writing the necessary code properly and plugging it into your existing site is your headache.
If you get stuck over specifics I'll be happy to help (assuming I'm online) but a walkthrough based on a table structure is more or less an impossible task for this forum.