The additional cost is paid when you pay the rest of Fireball’s costs, even though that text appears second on the card (to make the card easier to read). The additional cost doesn’t count toward Fireball’s converted mana cost.

If, for example, X is 5 and you choose three target creatures, Fireball has a total cost of (even though its mana cost is just ). If those creatures are all still legal targets as Fireball resolves, it deals 1 damage to each of them.

You may cast Fireball with zero targets, regardless of the value chosen for X. If you do so, it will not be a targeted spell, and no damage will actually be dealt when it resolves.

Fireball's damage is divided as Fireball resolves, not as it's cast, because there are no choices involved. The division involves only targets that are still legal at that time.

You can target more than X creatures. However, if the number of legal targets at the time Fireball resolves is greater than X, none of them will be dealt any damage.