section 179 deduction

I placed a new Chevy Tahoe for use in my business. It's used over 50% of the time for business. I'm being asked by Turbo Tax how much of a 179 deduction I want to take. I have no idea what to put in this box. How do I figure this out. Also, what about "special depreciation allowance? YES or NO?

I also want to make sure the maximum amount of the unused deduction is carried forward for future use.