Instructions
Select a group of units with your mouse. You can place any unit in the game into a group by clicking and dragging your mouse over all the units you wish to add to the group simultaneously.
Press and hold the "Ctrl" key on your keyboard.
Select a number key between one and nine while still holding "Ctrl." Release "Ctrl" after pressing the number. For example, holding "Ctrl" and pressing "1" on a group of marines binds them to the "1" key. Pressing "1" on your keyboard automatically selects those marines.
Press and hold "Shift" and the number key you assigned to a previous group on a new group of units to merge them. For example, select a group of reapers, hold "Shift" and press the "1" key. Now when you press "1" both the marines and the reapers are selected.
Click on a building to access it. For example, if you want to make combat units with the Terrans, click the barracks.
Press the key on your keyboard that corresponds to the unit you would like to make. For example, with the barracks selected, press "A" to make marines or press "G" to make a ghost.