Can you add an interest rate on small claim? ?

An ex-friend of mine has failed to repay money that he owes me. I made him sign an installment contract (to make paying it back easier for him) and he breached it. Now I am filing a small claims against him. I was told I can add an interest rate starting on the first day of the end of the contract that he failed to comply with. Is this true and if so what is a reasonable interest rate?