Well, first of all you would have to show that this was a false statement of fact. So, the statement cannot have been an opinion, and it must have been false. Example:
"I think Jim is a moron." is not a defamatory statement, because it clearly is an opinion.
"Jim stole $ 5000 from his dying grandmother" is a statement of fact. If Jim's grandmother passed away before he was born he could easily prove that it was false, too. Even a statement like: "You know, from what I hear everywhere I think Jim has stolen $ 5000 fromhis dying grandma" can be a false statement of fact, though it appears to be an opinion, if for example this guy made it all up and never "heard" anything.
The statement then must be derogatory, it must be such that the person's reputation in the opinion of a larger segment of the population would be damaged.
Example: If Jim is a member of a hard rock band and you tell the others: "I know that Jim really thinks country music is the best" this might put him down in the eyes of his peers, but probably not in the eyes of a large segment of the population, so it would not be derogatory.
But if you say: I know that Jim sleeps with his teenage daughters who are under 14" this definitely would put him down in the eyes of many people and therefore would be derogatory.
Then, when you have established this, you would want to look for fault. It is easy when you can show that the person made the statement intentionally like: Joe wants to get Jim's girlfriend, so he tells her: Do you know that Jim sleeps with underage kids?" But you don't need intent, it suffices if you can show negligence (unless the defendant is some public figure). Example: Someone told Joe that a co-worker in his department was arrested for underage sex. Jim did not show up for work today. So Joe goes around and tells all that Jim was arrested for underage sex. In reality, Jim was sick at home and it was Mike who was arrested. Joe was negligent because he did not attempt to find out what really happened to Jim and who really was arrested.
So, when you have all these elements, you need publication. Publication means the statement must have reached at least one other person than plaintiff or defendant. Example: Joe tells Jim: I know you sleep with underage girls! This is not defamation, because nobody but Jim heard this. But if Jim is standing in a group when Joe says this and they all hear it, it is publicized.
Once you have established this you look at privilege: A privilege can exist in many situations, for example is an employer privileged in statements if they concern the interest of his company .Example: Supervisor Joe tells supervisor Mike about Jim: I am sure Jim steals spare parts from our shop. Even if it was false, Joe was privileged to say this to Mike, because he acted in the interest of the company unless he clearly abused the privilege, which would be if he knew it was wrong and wanted Jim to be fired because he hates him.
Then you look for real damages. If your co-worker sends an email to his girlfriend saying: "I know that Joe takes some spare parts home from our shop every night", which is false, and she thinks "so what? I don't know Joe, I don't care about Joe" and forgets it, then Joe did probably not suffer much damage. He would only be able to get nominal damages. On the other hand, if his girlfriend actually is the manager of a company where Joe just did apply for a job and Joe doesn't get the job because of this, he might claim damages.