The Alfred the Great Tutorial match in Rise of Nations, gives you the basic perspective in creating units for your nation's military. Your job is to turn back the Viking hordes (represented by Germans), away from Britain, and liberate the City of Ethandun.
In order to achieve the mission goal, you must follow the objectives and instructions, commanded by the game narrator. You will likely receive something like this: build a Barracks, Stable, or Siege Factory, create more Longbowmen, train two Javelineers and Longbowmen, construct a Lookout to watch enemy movements on the border, have 3 Light Horses drafted, have two Catapults built, etc.
The British national borders are stretched to the thin, and its army's effectiveness cut short. You will have to train a new army to retake the land from the Vikings. You must reinstate your fighting capabilities, by mustering the all manpower you can possibly have. Rearm yourself by constructing a series of military buildings, defenses, and structures. For your first instruction, you must survey the Viking movements and business at Ethandun by constructing a Lookout on the edge of the forest border, only close enough so they won't attack. Then, you will be guided in the process of unit creation. Once your army is rallied and trained, send them over into Ethandun to render the Viking army stationed there beyond repair, and reclaim the city, its marvels, and people. Thus, you are victorious, and that concludes the Tutorial from the Learn to Play Menu in Rise of Nations.