What should I do after being fired for pointing out that my time-sheet was being altered without my knowledge?

You need to get your evidence together that shows that they altered your time card. You make copies of it, you show it to the dept of labor. File an official complaint. File for unemployment too and show them this. Then, GET A LAWYER! Sue them, because that is illegal and they should not get away with it! Hey, file a police report, altering time is fraud.