If you have not played a previous TOAW game before?
First of all check out the 3 videos that will be accessible from the game's launcher. They will not teach you all the rules and intricacies, but they will get through the main points, and on how to get started with the UI.
Then you can check out the Tutorial and the 2 other learning scenarios in the "Tutorials" scenario folder. These will have extra documentation accessible from the scenario selection menu (like many other scenarios, by the way), which will offer step by step instructions and advice, so you can learn a lot.
If you want to just jump straight in, which is quite possible (and how I started with the series back with TOAW Century of Warfare), "Arracourt 44" in the WWIi West Front folder is a tiny scenario for when you only want to move a few units around.
Also a classic starter scenario (and also used by the Tutorial scenario as I recall), is Korea 50-51, in the "Modern - Asia" folder - it shows off a lot of the game, but it is fairly easy to manage.