How do you get a judgment on someone?

You should be a little more specific. Do you mean that you want a court to order this person to pay you a certain amount of money? You would have to commence legal proceedings against this person. The easiest way is to hire a lawyer to do it, however that can be expensive. Most jurisdictions have a "small claims court" where you can represent yourself if the amount you are asking for is below a certain threshold. You can check at your local court.