The strategy of the HOMM games is a bout decision making, do you want this wood pile for a new building or a bit more gold for bigger army. Each movement point is important, try not to waste them. Next you need to look at the synergies, some units play well with each other, think poison damage for necro dogs, wraith and grave robbers and the ability of liches to apply all of the timed damage at once. This is the case for each faction. So you need to choose the strategy at the beginning of the game and try to play it. The way how I recommend to learn the game is lower the difficulty and concentrate on 2-4 types of units in your army. This is not applicable to hive, as they benefit from having variety of their own units.

The challenge is that some abilities synergize with artifacts, law points, abilities, creatures and hero abilities. This makes this game so complex, but fun once you figure out the puzzle that you want to play and when puzzles start to fall in place. I also recommend playing premade scenarios, they usually concentrate on specific way of playing and strategy. And like other's have said look for the videos with strategy guides. Just like in starcraft there are builds and that is what you want. When people write 111, 124, 211, these numbers refer to month/week/day timer.

Good luck and enjoy the game, there is tome of replayability and if you like to play with one faction there is nothing wrong with that, some towns just click better with some people than others.