Questions about the gameplay of Heroes 6.

Heh, hi. If anyone has the time and inclination, please add some information about the game in your replies. I'm not very good at Heroes, and I want to improve my skills a bit. There's just too much text to read at once, especially when I don't know all the words. I don't understand how the magic works, like Necromancy, when to use it, and on whom. I tried reading a guide about the Mage on this forum, but I didn't understand much. Okay, so sacrifice a unit, and the main units are Lich, Ghost, and Necromancer. That's it. I don't understand anything else, what damage they do, what happens? How do we kill enemies? And what is a stack? I understand it's our unit, but it can contain any number of units. Regarding castle development, here's how I think: "Why do they take the Hall of Heroes on the first turn? They're just wasting money. What will one hero give them, especially since it's expensive to buy? It's better to buy a building with warriors, like Ghosts, to become stronger and pose a greater threat. Why have more than one hero on a small map?" That's roughly the situation. Please, no harsh criticism :) Thanks, friends!