how is it possible to use carbon dating to determine the age

how is it possible to use carbon dating to determine the age of a fossil if the among of Carbon-14 has already decayed?How do you know what the initial amount of Carbon 14 was?Show the beta decay of a carbon 14atom.
大意就是 怎么用炭14去确定一块化石的年龄（炭14 已经退变了）你怎么知道原量的炭14是多少?