I am new in web development how should i start developing web site in PHP???
If you are doing it just to learn PHP, then w3schools is a place to start in order to learn some of the basics. If you are doing it because you would like a working website sooner rather than later, then I suggest that you use a Content Management System (CMS) instead. If you aren't already familiar with setting up an application in a web environment, then this is knowledge that you will need to have to test and run php programs anyway.
Suggest that you set up a local test environment using one of the available (LAMP) packages such as Uniform Server, XAMMP, WAMP or EasyPHP. These are open source packages that you can download and install on a Windows PC. Once you have that set up, you should be able to create a "Hello World" PHP program and run it under the local server that you have created.
If you have some programming experience in another language, then I would suggest that you download a few relatively simple PHP programs (go to Hotscripts or search the Web) get them to run in your local test environment and then examine the code and try to understand how they work. I also suggest that you download the PHP reference manual and keep it handy. You will also want a development IDE that can do syntax checks for you on PHP code. There are a number of them out there. You can try the PHP version of Netbeans. My site has some links for some of the PHP tools (see link below this post).