You have to prove that you lent the money and your friend is refusing to give it back. If it is less than $500, you can drag his ass to small claims court. If it's more, you need to serve him a notice by sending a certified court order. All you need is his name and address. A lawyer can prepare it for you. When he receives the court order, he must appear at court. Do you have a written agreement that your friend borrowed money from you? If no, then SMS him asking for your money. write the amount also in the sms. If he says, NO, I won't give your money, that's a proof you can use in the court.
But, I believe it won't end up in court. He should be ready to settle outside of the court. Good luck!!! How much did you lend anyway???