You need to sue and get a judgment first. Then you can sue for garnishment. Take all of the documentation that you have to prove that you made a loan.
[This communication is intended as general information and not specific legal advice, and this communication does not create an attorney-client relationship.]
I agree with Mr. Goldstein's answer. First sue him and obtain a Judgment and then you can collect on it by garnishment or by siezing his bank account or anything else you can find.
This is a common misperception about the role of the court. The court acts as a referee between parties. Therefore, the court will not garnish for you. However, if you wish to collect this debt, a small claims action would be your best bet. You should file the action, and bring all the documentation regarding the promise to pay and the amount. You should do so fairly soon as the statute of limitations is four years for an oral contract.
If you are able to prove that someone owes you a debt, the court will award you a judgement. A judgement is a court's order that you are entitled to collect the judgement. Once you have this order, you may then file to garnish the debtor's wages. You must file the necessary paperwork, serve it properly, and then file the necessary orders to have it paid to you. Make sure you keep an accounting to insure you are not paid money in excess of the judgement.
If the debtor earns more than exempt wages, you will receive a portion of each check to pay the debt.