If you use the rules in the built in library, or those supplied by other users, then you won't need to write code. But CGA allows you to write your own custom rules, and that is the fun. (at least for me it is).
You don't need to have programmed before. CGA is not a typical language - it is made for creating geometry. Rather than learning to write code, the first step is learning the basic operations available in CGA. Some basics are extrude, color, split, comp, and the roof operations (if you are making buildings). Beyond knowing what you can do with CGA, is to have shapes to apply the rules to. What type of input shapes do you have? Or will you be drawing or modeling shapes inside CityEngine? Can you explain what you are trying to do? Or is there a goal in mind?
Also, Python is not used for rules like CGA is, but is used for automating workflows, so you don't have to know anything about it.
