How do you build in starcraft

It is different for each race:

Terran: Select an SCV and click the Build option, then select what you want to build. Move your mouse to where you want to build, the building will be entirely green if you can build there. Click down, your SCV will move over there and begin construction. It cannot do anything else while building.

Protoss: Once agian, select your Probe and click build. Every other building besides Pylon and Nexus requires you to build in the blue area. The blue area is around a pylon. So move your building into the blue area of the map and click. Your Probe will start a warp-in, then you can instruct it to make other moves.

Zerg: Select your drone, click build. Almost every building requries you to build on the bed of purple biomass called "creep". Simply hover over this area so that your entire building is green.