On the right in the corner is the building menu, where you select a building, press LMB and put it in the right place (the building will be green). Also, each building requires a certain amount of resources, which is written on the panel at the top right (in the corner). The building with the image of the soldiers is the barracks, you can build them and by clicking LMB on the barracks you create troops. Also works with tanks, but they require microchips. They are made in chip factories. To give an order to soldiers or tanks, press RMB and they will go in the direction you clicked.
Mines must be connected to the surface! They must have access to it in order to work stone and metal. That's what tunnels are for(Press T to buid tunnels)