Confirm everything before relying on it. Many state's laws provide that you cannot lend money at an interest rate in excess of a certain statutory maximum. Many states have had lower limits in the past. Further, in most states a late charge or other fee exacted from someone who owes another money is also counted as interest.
Banks have separate rules.
To charge more you must get a banking, pawnbroking, or whatever license. This also means that special kinds of loans, like those from pawnbrokers or small loan companies are not stated. That's the rate that final judgments bear. Usury is a complicated area of law. Transactions that a person would not consider to be affected by usury often are, for example, repurchase agreements, or sales with an option to repurchase are often found to be loans.