I want to make a simple GUI Application. Can anybody give me source code of a simple GUI Application? So far, I have been competing in Programming Competitions (I have been using C++ for it) and now I want to get started with Application Development.
To make simple applications in Ubuntu, you'd possibly want to look at the Ubuntu App Developer API.
On this website, you'll find a plethora of resources for application development on Ubuntu and you'll see that, under "Programming languages" your knowledge of C++ will be well placed.
If you must see source code, there are some examples under the Ubuntu App Developer Cookbook as well as external links. This link has been removed as it is outdated.
Finally, you may also want to check out Qt for further reading.
Since this post first went up in 2013, with new versions of Ubuntu being pushed to stable, it seems the above is now outdated.
To make some simple applications (GUI or not), try the Ubuntu SDK. The Ubuntu SDK is based on Qt Creator, hence all simple GUI applications should probably go through this now.