Rule 118.3 · Costs
A player can’t pay a cost without having the necessary resources to pay it fully. For example, a player with only 1 life can’t pay a cost of 2 life, and a permanent that’s already tapped can’t be tapped to pay a cost. See rule 202, “Mana Cost and Color,” and rule 602, “Activating Activated Abilities.”
Subrules
Paying mana is done by removing the indicated mana from a player’s mana pool. (Players can always pay 0 mana.) If excess mana remains in that player’s mana pool after making that payment, the player announces what mana is still there.
Paying life is done by subtracting the indicated amount of life from a player’s life total. (Players can always pay 0 life.)
Activating mana abilities is not mandatory, even if paying a cost is.