Battle tactics against creatures on the map.

The problem is that mana is constantly an issue =\, there’s never enough to constantly support my units and for battles. And unfortunately, Orcs don’t have any initial magic. I don’t know why, but I’ve encountered this problem, and I often lose because I either run out of a couple of Ogres or have to deal with a Harpy, and I’m tired of the constant mana shortage. Maybe I just lack experience, so I’m looking for advice. I’m interested in learning about early hero builds and what castle development and unit focus would be best, especially at the beginning of the game.