1. It depends on the availability of resources and the length of the game. I enjoy long games and try to build resource-generating structures first. People need money more than any other race.
2. It 100% depends on the map. The longer the game, the stronger Humans become.
3. Humans are strong against everyone, and vice versa. I attack those I can defeat with minimal losses. The exception is that I try to take out the lumber mill and mine as quickly as possible, regardless of losses. Fortunately, they are usually taken without losses. As my army grows stronger, I try to take out the mines as quickly as possible.
4. It depends on the circumstances.
5. There is a dedicated thread for this.
6. The main strength of Humans is training. If the final battle is expected to take place in a town (usually against AI opponents), I train monks. If it's in the open field (usually against Humans), and there is enough money, I train knights. I try to level up the secondary hero with Skilled Counterattack and Experienced War Leader to reduce the cost of training. Troop gathering is also good.