Think of citric acid as a powder form of white vinegar... Its number one use is for descaling. Derived from the acids in citrus fruit (no surprise there, then)
Boil your kettle and unplug. Depending on how gross your scale level is, put about a third of a box of citric acid in the just boiled water. Enjoy the fizzing. When it's clean, rinse out, reboil and empty a couple of times. Sparkle, sparkle.