Dear Dipti,
While resigning from any company, you should keep one thing in mind. Don't say anything negative about the company, even if it is not a good company. The resignation letter should be short and sweet, with a genuine reason for leaving without affecting the image of the company.
Below is a copy of a post I found when I searched on CiteHR supporting my views:
Resignation is something that people do for two reasons. One, when you get a better opportunity where growth is perceivable, and the other when you or the management cannot see eye to eye! If the resignation is for the first reason, there is no problem. You can thank them for giving you an opportunity to work in that organization and for teaching you many things, enabling you to take on more responsibilities and career enhancements.
If the resignation is because of the second reason, even if you are frustrated for any reason, you cannot express them in writing the resignation letter. Very plainly, you can write that due to personal reasons, you are resigning from the job. Because, as other friends have put it, the world is small, and we will meet sometime or the other. Instead of turning this way and that, not able to face the other person, it is better to resign and leave without any hard feelings. I am sure, from my experience, the other person will remember you and will have better things to say about you in the future. This will all happen because of the ego that every one of us has. But while leaving, in a polite way, you can explain that you are not at fault and also share the circumstances that lead to this resignation.
I hope this reading will be beneficial for you.