What can I do if I purchased a car and now realize that the salesman misrepresented the cost?

I was given an out the door price of 35900. So I asked what other charges would be added on and he replied dmv fees and any other options I chose. I asked if sales tax was included and he said yes. Well it was late at night and I didn't really look carefully at all the charges. Sure enough they added on the sales tax and I signed and drove away. What can I do in this situation? I know he clearly said tax was included in the 35900. I haven't called them yet because I want to know if I have any options and if I have any rights, so I don't sound like a fool when I talk to them. Thanks.