Find a good hold down spot. Make money, get best perks and good guns. If everyone has good guns u can stay in hold down spot. If not then you'll probably have to run trains somewhere open. Also find friends that are good. 9/10 randoms are terrible at zombies, I don't know why but they are.
playing zombies is all about mindset, you must first get over that small spike of adrenaline you feel when you have a horde in ur a** and be able to manipulate their movements with your own
Don't worry so much about killing them at first as much as finding a nice open space and running them around always keeping your eyes forward anticipating where your going to go so you don't wind up cornered, remember your faster than them
Once you have them nice and horded behind you then you manipulate them into a nice straight line and line up 10-15 headshots while always being aware of what's to your back, and don't get caught in corners