Use the creature types and mana value of the creature you sacrificed as it existed on the battlefield to determine what creature card you can find. Notably, if you sacrifice a creature that's a double-faced permanent with its back face up, you'll use the characteristics of the back face, not the front face.

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

Once you announce that you're activating the activated ability, no player may take actions until the ability has been paid for. Notably, opponents can't try to remove one of your creatures to stop you from sacrificing it.