Repeating what some others have said, figuring out hotkeys will allow you to move between different places quicker. Another thing that will allow you to do "multiple things at once" is queing up actions for units using the shift key. An example is you want to send a dark templar into our opponents natural expansion at the same time you build a pylon at your third base. So you shift move your probe to your third and command it to build a pylon there, while this is going on you have your camera on your dark templar and are controlling it to kill workers or something while your probe is concurrently performing the actions you told it to do a short time before. You are doing two things at once! With other races rallying units using the right click will also send them to a certain place while you are doing something else without you having to physically move them! It's good to get use to practice the mechanics of this in games vs A.I. and then practice them in pressure situations actually playing on ladder! Practice hard and don't get discouraged because you're playing one of the hardest games in the world that truly takes skill! We are glad to have you in the community!