When you enter the vehicle section, it should ask you all of the applicable information, including the information for depreciation (such as the purchase price of the vehicle). However, if you indicate you want to use the Standard Mileage Rate (rather than Actual Expenses), depreciation is built-in to that, so you won't ALSO be taking depreciation as a separate deduction.
If you decide to use Actual Expenses (which includes depreciation) and don't want to 'pay back' the deduction when your vehicle drops to 50% or less, then you would need to choose to use Straight-Line depreciation this year. However, if I remember correctly, TurboTax is NOT set up for doing that for vehicles. So if that aspect is important to you, you may need to go to a tax professional.
