How do i make zombies?

This is a very vast question to answer. But I can give you some guidance as you go forward:

First I need to give you a Bird's eye view of what is going on with your zombi game: A zombi is an object in a game that contains several parts: a. body and other parts (you need to make sure whether you want to have your game designed in 3D or 2D) which needs to be designed and attached to your zombi object. b. intelligence (also known as AI = Artificial Intelligence in game industry) which is basically the brain of your zombie

Knowing the knowledge in the above paragraph, you can start by creating your object first and come up with a simple code and design for it. I strongly suggest to forget the zombie for now and start developing your game by creating simpler objects than zombie. For example a cube instead of you (the player) and a cylinder instead of zombie. Once you have these 2 objects in your scene, you can add basic codes of these 2 moving to these objects. Here is some basic codes related to moving.

Once you are more professional in moving the zombie (or making it follow you), you can design your 3D models and replace it with the cube and cylinder objects in your game. Then of course you can make the other objects in your game such as the game level, etc.

In order to develop 3D models, there are a list of software that you can use: http://forum.unity3d.com/threads/107077-What-is-the-best-free-3D-Modeling-software-to-use-with-Unity

I hope my thoughts at least gave you some clue to start...

Have fun!