You must pay the minimum due. This usually includes a little bit of principle and lot of interest since auto loans are usually front loaded.
As far as paying any additional towards only the principle, you'd have to talk to your lender or read through your contract. With my loan for example, any additional payment I make is applied to next month's payment up to 2 months before being applied to the principle. However, if I call them and make a large payment (common during tax return season) it will all apply directly to the principle.
