How to get Linux API's
 

How can I get all the API's for Linux like we have for Windows.

You only need to install the kernel-devel-* package. It contains all the headers for the Linux API. If you have header files in "/usr/include/linux" then you have the kernel development package and that includes the API.
It will be described in the relevant man pages.

All what to say is about the Kernal.
I expect some API functions to Create windows,Sending messages such as we have in windows like CreateWindowEx(),SendMessage() for application development.

If I am wrong in my concept,please bear it.

If you want to create GUI apps for Linux, you can use GTK or Qt. Both of these probably came with your distribution (whether you installed them or not, they should be on your CDs). There are probably other libraries available, but I think these are the most used.

I use QT and kdevelop for my development. The QT toolkit provides all the high level support for GUI applications. Gnome is based on the gtk toolkit. There are others but these two are the best.