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WHAT I DO NOW?
You fix whatever causes the error. We have no way to know how you do that. We don't know what is in your .JAR file; we don't know how you tried to run it; we don't know what you're running it on nor what OS is involved; and we don't know what errors you're seeing. We don't even know if you have an environment that can run Java. There is no help we can give without a lot more information.
From your question, it sounds like what you should do is enroll in a class that teaches Java.