1. A game engine like Unity, Cry, Frostbyte, Blender (I believe it's still free) and there you can import whatever resources you already have.
2. Learn to code (mostly java, python, C++) because you need to bind the actions of the character to the environment.
3. Learn to handle AI because you don't want the NPCs just standing there like a zombie.
4. And of course, a better storyline.