In God of War: Ascension, Titan mode is the hardest mode in the game. Enemies will be harder to kill, and you'll be easier to kill, so it's gonna take some skill, and probably a few cheats in order to get through it.
To unlock Titan Mode, you just need to beat the game once on any difficulty.
Have fun, and may Athena watch over you!
If you didn't get a chance to pre-order God of War: Ascension, check out the God of War: Ascension DLC on Ebay, which is probably the only way you'll be able to get them.