You would enter the truck under Vehicle Expense in the business expense section for your self employed business.
The information you enter (date of purchase, cost, mileage) is used to calculate the allowable depreciation for your vehicle. If you take the Standard Mileage Deduction instead of actual expenses, depreciation is included in the standard mileage rate. So, after entering the mileage information, you will want to continue on to the screen where you are asked if you would rather us Actual Costs for the deduction and then you will eventually be asked about Depreciation options.
