I need a help in a C program

You should do it on your own, because that is what homeworks are for. Have you even tried to find anything with google for example - I doubt.
First of all, about the game itself: http://en.wikipedia.org/wiki/Sudoku[^]
Secondly there are also complete solutions out there, like this one: http://www.pauls-pc-repair.com/sudoku/index.php?page=showSrc&src=solver/source/sudoku.c[^]
But be aware: this solution might not be implemented according to the requirements you got. Since it ha it's "signature", any teacher could find it also, and than you are cut. So, my advice is to consult it, even compile it, but first of all work on it to understand the code, and than make your own implementation.

It doesn't work like that.

You try to do it, you get stuck, you ask a question about your specific problem.
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.

Try it yourself, you may find it is not as difficult as you think!