Welcome!
mainah covered most of the points, and I mostly agree.
TripleA is really good. I estimated it decreased the amount of time it takes for me to play a solo game (by myself, I do that a lot) by 50-90%. Playing by the board is fun though if you have the time.
Definitely play online against real humans. mainah’s right, I learned a lot more from those than solo games. Let me know if you want to play.
AI’s not great, they’re only useful after you’ve become moderately good for quirky projects.
For IPCs, I use the National Production Chart. It’s definitely prone to errors, hence why I like TripleA (computer does it for you).
Personally I started with one theater first to get a feel for the basic mechanics (I progressed from 1941, which I played a ton of), but progressed to Global quickly (possibly one or two games too quickly). If this is your first Axis and Allies game ever, if you want to take things slowly start with one theater.
Once I started Global 1940 I’ve never looked back (only a few times to test single theater strategies).