Hiring an intellectual property lawyer to do this for you is a really good idea when you don't know how to do it yourself. Assuming you are in the U.S, there things involved in this kind of agreement that are important to include but that most nonlawyers would never think to put in the contract. If it's really important to you to own all the rights then its worth it pay a lawyer a few bucks to draft a good contract for it. 

If you are not in the U.S. then you need to find a forum that handles the law for your country. This forum focuses on law in the U.S.