I have filled up the resuviar but it still shows that it needs water. it there a place on the top of the radiator to fill it up???

i have a 1999 Saturn SC1

 

what shows it needs water ? the dash light?

with the engine cold, remove the reservoir cap and start the engine. Then get out and see if there are any air pockets being forced out of the cooling system. You'll know this to be the case if the reservoir level drops significantly. Turn off the engine after a minute and put the reservoir cap back on.

Re-start the car. If the dash light keeps flashing, the sensor at the bottom of the coolant reservoir is likely bad. You can remove the reservoir and try to clean it, but more often than not you need to replace the sensor.