What are the requirements for a Wrongful Termination Lawsuit?

The first question is, do you have a written employment contract? If you do, if it sets out the reasons or procedure by which you could be terminated, you could only be terminated in accordance with it. If you were terminated in violation of the contract, you could sue for breach of contract.
If you don't have a contract, do you believe you were terminated due to your race, religion, national origin, disability, age over 40, or gender? Those are the main types of illegal employment discrimination, and you can't be fired because of these reasons. (Note that, to use a deliberately extreme example, that an older disabled African woman from the Carribean who is Jewish, could be terminated for unrelated reasons, like poor performance, insubordination, personality conflict with supervisors, etc.--just not because she is an a older disabled African woman from the Carribean who is Jewish. I use this as an example becaue it implicates all the different categories: you can be fired if you are a member of a protected class, so long as the reason for termination is not because of your protected class membership.) If you think think this was the case and that you could prove it, contact the federal EEOC or your state equal/civil rights agency.
If you were not fired in violation of a contract and were not the victim of illegal discrimination, however, there is no such as "wrongful termination," because when there is no contract and no discrimination, you may be fired for any reason at all--no reason, even simple dislike by a supervisor, is wrongful. All employment, in the absence of a contract, is employment at will: the emploer can let you go at any time, for any reason, and does not have to treat you fairly or respectfully or professionally.