In Magic the Gathering does all of the criteria for the card Decimate have to fulfilled for it to work or could you play it even if an opponent doesnt control an artifact ect?

Because it targets, it can only be cast if there are enough valid targets for it. So for a card like Hex, there must be at least 6 creatures; Hex cannot be used just against 5 or less. Likewise, there must be a valid creature, artifact, enchantment and land for Decimate to target, or it can't even be cast. (Note, the opponent doesn't necessarily have to have an artifact, if you need to, you could target one of your own. But it still must target one artifact).

This is different to non-targeting cards like Akroma's Vengeance, which destroys all creatures, artifacts and enchantments. Because it does not target anything, it doesn't matter if there are no artifacts in play, it can still be cast.