Okay so I am trying to build a zombie deck and this is what I have so far:
http://deckbox.org/sets/94320

So right now my concern is that I don't have a lot of creatures and I have very few low cost creatures.

This is pretty much the first deck I have made.

Any suggestions would be greatly appreciated.

Of the cards available to you I think this is a decent setup. I wouldn't be concerned that you don't have a lot of creatures; 20 creatures is a decent amount. You may want to consider swapping out a few cards and increasing your land count to 24 though. I would probably drop the 2 Cellar Doors, the Demon's Horn and the Dead Weight for 4 swamps.

After that I would heavily consider bending your all-zombie-creatures rule to make room for the Grave Titan you have. He's close, he spits out zombies; he's just not a zombie himself. Trust me, you want him in the deck ;)

If you DO make room for him I would also swap out 2 of your Ghoulraisers for 2 Gravediggers. Your 3 CMC slot is a little crowded and it's way better to choose the creature you get back instead of getting something at random.

Let us know how the changes work out!

I have always liked Zombies so i have made some research on them. +1/+1 to all zombies cards are nice. Death Baron would be my choice same as Cemetery Reaper. Phylactery Lich can also be a good choice as it is a Zombie and i have seen many players go with Phyrexian Crusader. Though i have never used him, Vengeful Pharaoh seems like an interesting card