2000 Opel Astra water for radiator

Hello there, I have had my car for 2 years now and lately have had to keep filling up the water for the radiator have never had to do this before and the gauge for the engine is going way up. I dont think the radiator is leaking and even when iI go a short journey the engine seems like it takes a long time to cool down. Its my first car and dont know much about engines