How do I create a virtual piano using C programming language?

You won't find any code that fits exactly what you want: and even if you could, that would be plagiarism, not "doing a project" - and most tutors are wise to that!.

We do not do your homework: it is set for a reason. It is there so that you think about what you have been told, and try to understand it. It is also there so that your tutor can identify areas where you are weak, and focus more attention on remedial action.

If you meet a specific problem, then please ask about that and we will do our best to help. But we aren't going to do it all for you!

But ... You don't sound like you have much development experience here, and this isn't a job for a complete beginner: you are going to have to learn to code in C for a graphics environment (in order to draw the pictures) which in C probably means learning how to code in Windows - which is decidedly non-trivial!
If you can, find a simpler project, or change languages: C# would be a load easier to do this in as the initial learning curve is a lot less "brick wall" like than coding in C for Windows.

So if you really want to do the work of a seasoned programmer you need to learn programming. This needs some weeks of intensive learning to acquire the basic skills. You also should consider a higher level language like C# or Java - depending on your target platfrom.

For showing something on the screen normally is the use of bitmaps the best and easiest way. In your app: a "non pressed key bitmap, and when pressed than exchanged with a "pressed key" bitmap. Like bricks in a wall you show an array of these elements.

When I search with Google I had found this simple but good keyboard example in some seconds. (You only need to set suitable bitmaps on the buttons) Shame on you...