How can I learn to manage multitasking when fighting in Starcraft 2?

I've never understood how to use hotkeys (for eg: 4-barracks, 5-starport etc up to 8). How do the professionals (or even good amateurs) manage using keyboard left hand and mouse right hand? I am not a great typist but an average typer.

What I am trying to say is: I am really unable to manage multi-tasking unless I use both hands on the keyboard. But while fighting, I cannot use 2 hands on keyboard, since mouse definitely needed on other hand. I am having problem especially while fighting even though I use all hotkeys (Grid hotkeys I mean )

Sorry if this question already asked, but before I ask already searched and I found some answers too like saying that we should use hotkeys etc..