Top up the battery cells with distilled water. You should put in enough to cover the plates by about 1/2 inch. Check to see that your battery is not the sealed type. If it is, you can't top it up.
Is the battery a fully sealed one cause if it is you dont need to fill it at all. If the battery has a strip on the top that you pull off or small round screw nuts that you unscrew to top up fluid then you need to buy the right type of water to top up the battery. DONT put normal tap water in a car battery it wont cahrge very well.