I want to make a site like facebook for my schools intranet does anyone know how i could do this ?

Judging by the general vagueness of your question, I am guessing that you don't have any prior knowledge of server-side scripting. Basically, you would need to use a scripting language in your web pages that can act as the go between for your actual web pages and a database of information.

The most popular language and database combination in use now is PHP/MySQL. There are many books on PHP (scripting language) and MySQL (database application). The reason for their popularity is the fact that they are both free.

Do a simple search on either of them and you will come up with a wealth of information. Unfortunately, these are much more complex than HTML editing.

Facebook basically uses a ton of forms created with HTML that users fill out. The information that they put into the forms (name, school, picture, hobbies, etc) is then transported over to the MySQL database using a little bit of PHP. The real area that PHP comes in to play is in displaying all that information in a visually pleasing way. The layout of the pages on Facebook are created using HTML in tandem with PHP (which asks for, and then interprets, information in the MySQL database).

I hope that it is clear that there is no short answer for such a question. I hope, though, that you at least have some idea of what you need to learn.

I think you can buy software to do that.

From scratch: you'd need a server and a web application in PHP, ASP, or some other language/framework. You'd need a backend: PostGres, MySQL, SQLServer, or some other DB system. You'd also need to handle security, etc. It would be a very difficult undertaking to develop a site like Facebook, which has many developers and is quite mature in its development cycle.