What evidence will I need to provide for a small claims case In Massachusetts?

First, there was a contract involved.  You loaned him $ and he agreed to repay it.  Even if you didn't set the terms for repayment, the law will presume that your friend had to repay you within a reasonable time. Second, as to what evidence you will need to provide, you should provide all the evidence you can.  Your testimony, the testimony of any other witnesses with personal knowledge (people who know about the loan only because you told them about it have no personal knowledge; people who know about it because your friend admitted it can testify as to his admission), copies of any repayment checks, copies of the check or other document evidencing the loan, copies of emails, texts, or letters between you and your friend with refer to the loan, etc.  Nobody can tell you whether the evidence you have will be enough; it all depends on whether, assuming your friend denies that he owes any money, the Judge believes you or him.  The more evidence you produce, the more likely that the Judge will believe you.