Welcome to the board.
To get right into your question: If you choose to sue them, you will want to contact a lawyer to assist you with this matter.
Now, I have a few opinions on this matter:
Before rushing into suing them (because that is a drastic step), contact your company’s Human Resources Department and talk to them about the problems you are having. They might be best able to assist you with your problem.
If your Human Resources Department can’t help you, then try to talk with your boss. Tell him/her of all these lies and how you can’t stand it anymore and ask him to do something about it (perhaps by reading them the riot act).
As for what you can sue them for. Defamation is really your only recourse, but then you have to be able to PROVE it was them. Other than your word vs. Theirs, is there any proof? The answer to that has to be yes.
But then, I would recommend courses other than suing for the time being.
One more thing:
You say that you never say “NO” to any of them. Why not? Start saying NO to them once and a while. You are doing THEM favours, and they treat you like crap? That seems unfair to me.