How to create PHP websites using a LAMP-server?

I have Ubuntu 11.04 and I have installed a LAMP server using the command: sudo apt-get install LAMP-server^

Apache was tested, when put http://localhost/ in the browser (Mozilla Firefox) the message was shown: It's working!

Now I'd like to know where can I find the DocumentRoot, where I can place my PHP-files. Before that, should I use a (PHP) program to create PHP websites? Is it installed with the LAMP-server and ready to use? I am new to PHP and I'd like to start learning it using Ubuntu.