How to learn the game? Play a lot
Also helps to play-by-email with some experienced SP warrior, you'll be beaten but you'd be able to see few tricks of the trade
But at first, tutorial and small battles against AI to get hang of controls would be the best.Note that you may purchase units for AI as well if you set the battle up correctly, you may use this if you want to try all-infantry or all-tank or whatever battles.
Note from my personal experience: Don't neglect the infantry! they're pesky buggers, fragile, slow to move etc. but they might be decisive. Took me a long time to begin to appreciate them and cost me good number of battles till the lesson sunk in