Borrowed money interest

[Hengbok81]

Hi.. please i need help... my really close friend borrowed $10,000 from me in July and he said he was going to pay me back in 1 year with interest.. a year passed and he's not paying me back.... I have a paper with his signature saying that he owes me $15,000($5000 interest) and I have 2 witnesses signature as well... will I win this case? or is it illegal to even make this paper to start out with?? some people say.. you can't win cause you need to have it notorized or have a lawyer in the middle agreeing to it and some people say I can win because I have his signature approving and understanding the consequences and 2 witnesses signature.... help!