Radiator

You need to have the entire cooling and heating system filled with a 50/50 mix of antifreeze and distilled water to keep it from freezing.
If it has only water in it now you should drain it out before you run it because straight water not only freezes but it does not cool as well as a proper coolant mix either. However, if you drain the radiator down as far as it will go there will still be some water in the engine so you would want to add in about half a gallon of straight antifreeze, then top off with 50/50.

Or take it to a shop and have them do a coolant flush. That would flush out all of what is in there and you get all new coolant and do not need to deal with the mess.

Your choice But if it is plain water and you live in an area that freezes get it done soon. Freezing water in an engine will destroy it 99% of the time.

Do not have enough money to get what done?

The mixture should be fifty percent water and fifty percent antifreeze. That will get the freeze point down to minus thirty five degrees. The people at the place you get your oil changed should have a simple freeze point tester. They should be able to tell you the freeze point in half a minute. It is good to understand if the freeze point is only good to, oh, minus fifteen degrees, and the temperature gets down to minus twenty degrees, the coolant is not going to freeze into a solid block. It will turn into a slush that will melt when the engine warms up.

It is even more important to understand an odd characteristic about antifreeze. While the right amount is good, more is not better. The lowest freeze protection is achieved with a seventy percent mix of antifreeze and thirty percent water. That gets it down to around minus fifty degrees. As the percentage of antifreeze increases from there, the freeze point actually starts to go back up. One hundred percent antifreeze freezes, as I recall, at around minus ten degrees. Water is needed to form a chemical reaction that lowers the freeze point.

To add to the confusion, simple, inexpensive freeze point testers have a major limitation. They calculate the freeze point by comparing the weight of the mixture to the weight of water. Antifreeze is heavier than water, so the tester will show straight antifreeze as having a real low freeze point, but in fact, you need some water to make the freeze point lower. That means as the true freeze point goes lower as you add some water, the freeze point shown on the tester goes higher. As long as you have close to a 50/50 mix of antifreeze and water, the inexpensive testers are pretty accurate in that range.

Antifreeze has anti-corrosion inhibitors and water pump lubricant additives that wear out in about two years. That is why we need to replace it periodically. If the previous owner ran straight water in the system, those additives were missing. The downside of antifreeze is it cannot hold many BTU's of heat. That means it inefficient at moving heat from the engine to the radiator. Water is able to hold a lot of heat volume, so it is real good at moving the heat. Another reason why you need both in the system.

If you have only water in the cooling system now, adding a cup of antifreeze is no more effective than using a match to heat your house. Some of the water needs to be drained so it can be replaced with antifreeze. The amount of coolant the system holds depends on the engine size. You did not list that, but in general, typical systems hold a total of about three to four gallons for four-cylinder engines.

You never said you had just plain water in it, so I will assume that it already does have some sort of anti-freeze/water mixture. Just purchase an inexpensive ($4.00) anti-freeze tester from just about anywhere, an auto-parts store, Walmart, a dollar store perhaps, etc and test the fluid in the cooling system. If it is already protected to -10f degrees or so and you do not live in northern USA, Canada or Alaska then you are fine. If it does not have the proper protection, then worry about how much anti-freeze you need to purchase and how much to drain out.