My temperature gauge is at it's highest and u don't know where to add the water

Either the radiator cap or if there's no cap on the radiator, then the overflow bottle should have a special type cap. In all cases, be very careful because the system is under pressure and you can get seriously burned by spraying coolant. If you get the cap off safely, make sure the engine is running when you add in coolant otherwise the cold coolant in a hot engine could crack the block. Good luck and be careful

If your gauge is at the highest there is a reason for it. It might not be low on coolant. It could be a bad thermostat, the cooling fans are not kicking on, plugged radiator or heater core.