Staffs

Pick the right template when you create your score. Or select the "instruments" indiviually at creation time. See the handbook (which I propose you quickly browse through):
https://musescore.org/en/handbook/4/setting-your-score

If you have already set up your score, and now want to modify it, you open the "instruments" view/dialog and set it up as you wish.
https://musescore.org/en/handbook/4/setting-your-score#changing-instrum…

As Andreas said, immediately after clicking the "New Score" button, do this on the New Score dialog.

First yellow, then blue, then red.