The first thing I would advise is to make sure that you get into good habits early, by that I mean you refactor when you need to, avoid programming on the timeline, make sure you adhere to best practices, format nicely etc It seems a lot of designers tend to be rather messy, dynamic sorts and when they then get into coding produce a house of horrors.
The second thing to do is to learn the basics of programming theory and not get too caught up in the intricacies of Flash to start off with. Some things you should know:
- What is a class, why use classes?
- Difference between static and instances
- Understand various datastructures like Arrays, Hashtables, Binary Trees, Linked Lists etc and know which suits which purpose best.
- Understand inheritance and be able to spot when to use it
- Understand polymorphism
- While not applicable to ActionScript, be aware of Generics
- Have an understanding of memory management, better yet have a look at an unmanaged language like C++. Understanding how memory is allocated and the purpose of pointers. Doing so improves ones knowledge of what Flash is doing behind the scenes.
- Be aware of design patterns
Hopefully I haven't scared you off. Now in reality you don't need to have knowledge of all of these things, you can still code and experiment and produce something. But having knowledge of the above list is essential if you really want to get into programming and be able to write good code on large projects.