You will enter the price of the truck, not just the down payment. The total value is what may be depreciated.
Only the interest on the loan is expensed.
Payments on the loan are not deductible.
You may have depreciation options. If you want to offset the monthly payments year after year, you may opt for the regular depreciation. If you want to offset more expenses in the year you purchase the truck, use bonus depreciation or the 179 Deduction, if you are otherwise eligible.