It depends on what you're using it for.
If you're using it as a business expense, then you take the mileage on Schedule C. You claim that by going to Business Expenses>Business Vehicle Expenses on your Business tab.
If you're using it as a car to check on rental properties, you take it as a deduction on Schedule E. Go to Rental Properties and Royalties>Vehicle Expenses on your Business tab.
If you're taking off sales tax paid on a new car, you take it as a deduction on Schedule A. Go to Deductions & Credits>Estimates and Other Taxes Paid>Sales Tax.
If you're taking off the registration fee, you take it as a deduction on Schedule A. Go to Deductions & Credits>Cars and Other Things you Own>Car Registration Fees. Depending on the state in which you live, you may or may not be able to deduct it.
