The learning curve for AutoCAD is about 10,000x steeper than learning g-code.
g-code is really quite simple. It's just XY coordinates, with codes to tell the machine how to get to those coordinates. For a router, all you really need to know is 4 basic g-codes.
G0 - move at rapid rate
G1 - move at feedrate, specified with F (F100 = 100ipm)
G2 - clockwise arc
G3 - counterclockwise arc.
There's a button in Mach3 which brings up a list of g-codes, with explanations and examples. You can also find this info in the manual, which has an entire section on g-code. If you want to get advanced, most recommend a book by Peter Smid.
Personally, I'd recommend learning CAD and CAM. Download the demo version of Vectric's V Carve Pro, and work through all the tutorials. This is the easiest method of creating complex designs, and turning them into g-code.
While it's good to understand g-code, I don't recommend hand writing it. It takes a long time, and handwriting will inevitably have errors from typing mistakes, which are unavoidable.