Payments are applied first towards unpaid fees, then towards accrued interest, and the remaining amount is applied to the principal balance. Interest accrues daily on the outstanding principal of the loan. This means that towards the beginning of the loan, more of the payment is applied to interest than at the end of the loan when more of the payment is applied to principal.  As you make consistent payments over time, the portion applied to principal will increase and the portion applied to interest will decrease. 

To see how much unpaid interest is currently accrued on your loan, review the loan summary page on your dashboard.