First you would die, either from asphyxiation or hypothermia. Next you would begin to dry out. There is no liquid water on the surface of Mars and little, if any, water vapor in the atmosphere. Your lifeless body would become dessicated like an Egyptian mummy. Finally, not that it would matter terribly, you would contract a very nasty sunburn.
Radiation levels are so intense that they probably sterilize the uppermost layers of Martian soil.
The next time you visit Mars, take a space suit. Undoubtedly present-day Mars is not a congenial place for life as we know it, but it may have been friendlier in the distant past. There may have even been an ocean . To allow that to happen, the atmosphere must have been a lot denser than it is today. A dynamic molten core gave rise to a global magnetic field that protected Mars from the ravages of the solar wind and powered tectonic activity in the Martian crust.