Magic The Gathering: declare x?

Question

Do you have to declare what x is in the beginning of the game, or when a card asks for x. If so, can you make x any number?

Answer 1

X on a card is 0 until set and cast. For example Fireball is X and one red. This means you spend one red mana then you can decide how much you want to spend for X, any colour mana can be used (unless indicated).

Answer 2

x is the variable.

Answer 3

Whenever you play a spell or ability with X as a cost, you announce what x will equal when you play it. X can equal any amount of the resource that you can afford (usually mana, but sometimes life or sacrificing creatures).