Do I have to use only coolant for radiator or mix coolant with water, if so what percentage of coolant and water should be mixed?
As @Larry has stated straight water or antifreeze are not ideal coolants when used alone. When mixed in a 50/50 ratio the boil/freeze point is ideal for all but the most extreme climates. If you are unsure of how to achieve this ratio. Safely empty and capture as much of the coolant as possible. This may involve removing a drain plug in the radiator along with the block. Although it is more expensive, premixed coolant is available which will allow you to refill the system with the proper 50/50 mix. An alternative would be to get an empty coolant container fill it half full of water and fill the rest with straight antifreeze to achieve the proper mix. pour it into the cooling system, repeat the process until the cooling system is full. Depending on the vehicle, filling the system completely can become involved. As the system is refilled air is trapped and it must be burped. In the simplest systems running the engine does this. In extreme cases it must be vacum filled.