Press "." (period) to focus the camera and choose an idle villager.
This is the most important hotkey in the game, that lets you quickly assign idle villagers to work and be as efficient as possible.

Press H to go to your town center. This is very useful to make sure it's not idle. Additionally, you can press A when your town center is chosen to quickly create a villager.

Choose a villager you want to build with, and use these:
A , Q = house
A , A = farm (the easiest to remember, just double-tap A)
A , E = mining camp
A , R = lumber camp

Choose a unit and press G, then click a building/ram/ship/anything else you can garrison inside to make the unit garrison there.
Useful for defense against raiders, or to protect your boar lure.

Choose the unit(s) you want, and press ctrl + any number from 1-9 to mark the unit(s) with that number. Then, to quickly choose the units you marked, press the number you chose, and that should choose the units. To foucs the camera on them, press space.
This is very useful to quickly go to your scout (I always mark it with "1") or your army.

Choose a group of soliders and press TAB, then click behind your enemy's army.
Your soliders will now move towards the point you clicked, but will stop to attack any nearby enemy units.

When choosing a group of soliders, you can make them stand in a certain formation using the keys Z,X,C,V. The most imporant and useful formations are:

• (Z) for the default line formation
  
• (V) for a V-shaped formation

Tip:
When in the default (Z) formation, press V to quickly & effectively dodge an onager shot.