Fill it with a mixture of half white vinegar and half water, and leave to soak overnight. (Make sure the kettle is unplugged and there's a note on it so nobody uses it accidentally!) In the morning, the limescale will come off easily. Rinse thoroughly to remove any lingering vinegary odours, and boil up fresh water after the treatment.
There are also chemical descalers available to buy, some designed specifically for metal kettles and others for plastic models. You should always check if they are suitable for your appliance. Quickshine Kettle Descaler Sachets can be used on all kettles and are environmentally friendly.
You should be descaling every four to eight weeks to keep your kettle clean — don't let scale build up, as it's more difficult to remove the longer it's there.