Hello Sarah,
If you have QuickBooks Premier edition you would set it up in the Loan Manager window following all prompts. The payments would then be able to breakdown each payment by principal and interest automatically.
If you are using QuickBooks Pro you would set up a loan when entering the deposit or by going to the Chart of Accounts, choosing Account tab; new; follow the prompts. You can put the terms in the description field. You would need a payment schedule or Loan Amortization Schedule to split each payment by principal and interest.
Loan Amortization templates can be found in Microsoft Office or Excel programs.