How would I go about creating a new social networking website like facebook, myspace, etc.?

[Wil]

You will need a few things.

You will probably want to hire a company like mine to do it for you, but if you don't want to then you can start by looking at Content Management Systems like Joomla or Drupal.

Joomla is more refined and easier to understand allowing for the options such as you'd have on the sites you'd mentioned.

Drupal is more powerful and has more capability but as this increases so does confusion.

Ruby On Rails is a good programming language to start with if you want to program one yourself.

Either way check out the resources below. All are open source solutions, but you will require a basic to intermediate understanding of PHP, CSS and even HTML would help.

http://www.Joomla.org
http://www.Drupal.org
http://Rubyonrails.org

If you need help IM or EMAIL me, I will also check here to see if you need more advice.