Hi, if you are religious, then your religion [ eg 10 Commandments] will tell you. Most simply, your conscience will tell you whether you are doing a bad or good thing.

I think that something that is good has positive effects, and intentions. If you do something to help someone or something and you did it because your intentions were to help them rather than for some sort of personal benefit then it is good. If your intentions were bad then it was bad, even if the outcome was good. Good and bad varies between cultures, religions, and personal convictions. Different peoples views of different things being good and bad causes a lot of problems. For example the United States believes that they are fighting the war with Iraq for a good cause, but Iraq also believes that they are fighting the war for a good cause. Who is actually good and who is bad? I don't know.