Most Magic players aren't willing to play the hapless victim at the first sign of adversity, but Zombies are notoriously relentless. As long as you have a Zombie on the battlefield, you can cast Gravecrawler from your graveyard. Geralf's Messenger is ravenous, biting a chunk out of your opponent's life total once when enters the battlefield, and then again when it dies and comes back bigger thanks to its undying ability. And Cemetery Reaper not only makes your Zombies bigger, but it can also make new ones out of the fresh corpses in your graveyard and your opponent's.
Zombies wouldn't be terrifying if all they did was shuffle around and come back for more. Skinrender has a toxic touch, putting three -1/-1 counters on target creature as it enters the battlefield. Between it, Go for the Throat, and Dismember you have a good shot of killing every creature your opponent leaves back to block.
Your main deck focuses on playing creatures and killing creatures so that yours can attack. Your sideboard is full of discard like Distress and Despise to use against opponents who don't play many creatures and would rather try to slow you down with spells like Day of Judgment or powerful planeswalkers. If your opponent's deck is good at killing your creatures, bring in another Ghoulraiser to ensure that you never run out of threats.
When you're ready to customize this deck, there are multiple directions you can go. If you enjoy crushing all your opponent's creatures, Call to the Grave from the Magic 2012 core set might tickle your fancy while it saps your opponent's will to live. The black enchantment forces each player to sacrifice a non-Zombie creature at the beginning of his or her turn. Prefer overrunning the battlefield with your Zombie horde as quickly as possible? Try adding some sources of blue mana for Diregraf Captain from the Dark Ascension set. It makes your other Zombies more lethal on the battlefield and on the way to the graveyard. If you do shamble down that path, the Avacyn Restored land Cavern of Souls can help you cast all of your Zombies no matter what color they may be.