how to write a code for different pages ?

to write a code .

If you want to write a code separately in different pages then you have to create different pages for HTML,CSS,JavaScript etc.

So you can create different pages and save them all with its own extensions like for HTML (index.html) for CSS(style.css) and for Javascript (script.js).

You can also use it in a single HTML  file by using tags. You will write code for styling with style tags. Similarly you can write code within for javascript within script tags and overall save the file with index.html.

Please make sure before switching to emulator you have to save all you code instead to using save option because save all can save all other documents where you have edited. 

to write code in html5 for app development makes you easy way to convert your websites to app easily today and intel xdk provides you to write once and then a single coded app can be used for android,IOS,windows,crosswalk android etc.
A typical intel XDK project, mainly corresponds to an app of a base of,

1. HTML5
2. CSS
3. IMAGES
4. JAVASCRIPT

This is your first project in xdk.
then you have to START A NEW PROJECT or you can import an existing intel xdk project in to the xdk.