Any X in the creature's mana cost is zero.
The mana cost you pay includes colored mana.
The creature enters the battlefield, so it will trigger enters-the-battlefield abilities even if you have chosen to not pay.
If you choose not to pay, the creature is sacrificed immediately. No player will get priority in between the creature entering the battlefield and being sacrificed. Sacrificing the creature this way will trigger any abilities that trigger when a creature leaves the battlefield.