If you haven't already, press Ctrl + O and go to Controls. That should give you a general idea. Try the tutorial as well.
To build, press a number on the top row of your keyboard. You can select bricks with B, and press Done when you finish to keep them in your "cart". Then, just click, use your number pad to move it around (or IJKLUOP; etc. if you're on a laptop), and press Enter to place.