If a creature card in your library or graveyard has {X} in its mana cost, X is considered to be 0.

If X is 10 or more, the creature card you just put onto the battlefield will get +X/+X and haste.

No player may take action between the time you reveal which creature card you'll put onto the battlefield and the time it gets +X/+X and haste if X is 10 or more. Any abilities that trigger as it enters the battlefield will be put onto the stack after your creatures get +X/+X and haste.

If you don't find a creature card with mana value X or less, creatures you control still get +X/+X and gain haste if X is 10 or more.