If you are itemizing, and want to deduct your car's sales tax, you can enter it into TurboTax here:

• Federal Taxes (Personal if using TurboTax Self Employed or Home and Business)
• Deductions & Credits
• You may need to click on Skip to see all tax breaks
• Scroll down to Estimates and Other Taxes Paid and click on Show More
• Sales Tax
• Start or update

If you mean the personal property taxes on your car, enter them here:

• Select Federal Taxes 
• Select Deductions & Credits
• You may need to click on Skip to see all tax breaks
• Scroll down to Cars and Other things you own and click on Show More
• Personal Property Taxes
• Start or update