Rule 118.5 · Costs
Some costs are represented by {0}, or are reduced to {0}. The action necessary for a player to pay such a cost is the player’s acknowledgment that they are paying it. Even though such a cost requires no resources, it’s not automatically paid.
Subrules
A spell whose mana cost is {0} must still be cast the same way as one with a cost greater than zero; it won’t cast itself automatically. The same is true for an activated ability whose cost is {0}.