Creating user registeration by

I have been searching on google about how can I add sigin in & sign up functionality to my wordpress website. I want my site to have some restricted area which will only be shown to registered users. But nearly on all links I found that they were using previously developed plugins to add this functionality. But I am interested in knowing how to develope such plugins.

So far I have not found a resource about such plugin development specifically takling about user registeration.

If anyone know please post links for tutorials.