My Car Is Running Hot. The Antifreeze Section Is Empty And When I Open The Radiator I Can't See Whether It Has Coolant Or Water. What Should I Put?

Actually being pressurized is what stops the water from boiling.
Either way there is probably a leak or a head gasket problem if you can't even see any in the radiator. Fill it with water, start the vehicle and let it run while slowly adding more to fill as it sucks the water through the system. Make sure you add it SLOWLY so as not to introduce air bubbles into the system which can cause vapor lock. Also squeeze the upper radiator hose to get the air bubbles out of it while filling

then watch where the water starts coming out of. If you can't see any water on the ground but its still disappearing, you have a blown head gasket.

Antifreeze stops the water from boiling and steaming out. Please use some antifreeze.

You might want to use just water at this point.  If it is low in the radiator and it has already emptied the reservoir there is a good chance you have a leak, and you don't want to lose antifreeze if it is leaking.
I would fill the radiator, then bring the reservoir up to the cold mark.
 
If it holds up OK, flush it and put the correct antifreeze/water mixture before cold weather.

As long as the weather is warm, then you should not have any problems.  You may have a hole in one your hoses, so don't waste antifreeze at this time.  If your car continues to leak  try to take it to one of many automotive franchises to get it checked out.

Put a mixture of 50% water and 50% antifreeze in both the radiator and the reservoir. Do this once the car has cooled down and then start it to circulate the mixture through the engine. Accidentally mixed? Whatever you put in the radiator will mix almost immediately and what is in the reservoir will eventually be pulled into the engine cooling system.