I need to top my radiator fluid.....but how?

First read your car's owner's manual. If you have a radiator cap both on the top of the radiator and on the coolant overflow reservoir, wait until the engine's cold and remove both. Fill the radiator itself to overflowing with 50/50% antifreeze coolant and put the radiator cap back on. Fill the reservoir to the cold fill line marking and put the cap back on. However, you need a radiator coolant drain and refill if you haven't done that in the last 3 years, so consider having it done at a repair shop.