You could start by reading the php documentation. The documentation itself contains a tutorial. After finishing the tutorial, you should be able to understand the rest of the manual and understand the examples in it. You could skip chapter 2 if you don't have plans on setting up you own server.
In case you want to set up your own server, you need to download the ff:
- Apache - web server
- PHP - hypertext preprocessor to be used with apache
- MySQL - (optional) database server. Most likely would need it later.
You can download them separately or use a package called xampp.
http://www.apachefriends.org/en/xampp.html
The php documentation can be read online or download it for offline reading at
http://www.php.net/docs.php
After learning php, you could start learning SQL, a language used in MySQL. Learning MySQL is essential since most php applications use it.