Start by placing buildings at your mouse pointer and pressing space to place them. Once you click on a building, its stats will show. Upgrading it will increase the Capacity, clicking on the mini-warrior near the bottom-right corner will train a person to do the job stated. The troops trained in the Training barrack are 3 times more powerful than those trained in the Military Command but cost more. Every 2 minutes from the time you start placing your buildings, opponent troops will come and invade.
Note: The instructions are very important! Read it all!