When I played this game ages ago for the first time, I was really looking for something like this as it would helped me as a newbie to know what to take care of to power up the commanders.

So here we go! You gain one piece of wargear...
...for having a 3:1 kill ratio with at least 200 killed enemy units
...for 1 province conquered
...for 5 provinces conquered
...for 15 provinces conquered
...for 2,000 killed enemy units (in total)
...for 5,000 killed enemy units
...for 3 conquered enemies (take their strongholds)
...for 1 defense victory (only played defenses count, not auto-resolved missions)
...for 3 defense victories
...for 5 defense victories

Get wargear every time you unlock one piece. Depending on whether you play your commander offensive or defensive, chose the best matching piece of wargear. I prefer the following: First piece should enable the commander to see infiltrated units, then power up the life and damage if you go offensive, or the life and armor if you prefer a more defensive play style.

Cheers!