The basics of ROM hacking is the place you should definitely start at - the idea is that by changing the data or code in the ROM of the game you can customise it with new things that you want. You'll probably want a hex editor to start with to do some basic modifications. HxD
is one such example (find one you like, there are thousands out there for every OS you can think of).
A hex editor is the starting place because it is a tool for changing the contents of any binary file (including ROMs!). For something to look at to begin with, try some text replacement. You can search for it and it's easy to modify/see your changes in the game. I have a notes document
I maintain for each of the AW games. I'd recommend having a read through RHDN
- they're an all-purpose romhacking community that has a lot of documents, guides, utilities and a bunch of things for romhacking all sorts of systems.
See this thread
or this thread
to get started.
How to create a custom CO and put it into the game
A custom CO is a collection of multiple things, so it depends on what extent you want to dive in. As an absolute beginner, I'd recommend *only* replacing a CO (Flak or Nell is a good candidate) with what you want to do and play around with changing the stats before you commit to something more difficult than building art for something.
A complete CO needs a whole bunch of things:
- Day to Day Stats Table Entry
- CO Power Stats Table Entry
- Super CO Power Stats Table Entry
- Graphics & Animation Table Entry
- Graphics & Animation Table Entry (CO Power)
- Graphics & Animation Table Entry (Super CO Power)
- AI Rules Table Entries (Land, Land/Air, Land/Sea and Land/Air/Sea maps)
- Custom code for any CO Power effects or other mechanics that the game doesn't support by default (this will probably require some learning!)
- Dialogue Faces (Happy, Sad, Neutral)
- Small Face (Battle Animations/CO Dossier Pages)
- Small Face (Info-box with funds/COP Charge)
- Super CO Power description
Each CO has their own theme...but hacking this yourself is exceptionally difficult. Having musical knowledge also helps, but I have almost none and can't help you with this aspect.
A playable CO to test with only really requires the stuff in the gameplay category to be correct, but there's a lot of other stuff going on for a "finished" product. I'd recommend starting with something really basic (text editing is the best, then you can ask questions as you don't understand stuff, it's also the least technical aspect of romhacking).
For the record too, I started learning to rom hack in high school.