How to format score/fit to page?

There's a whole chapter in the handbook for this, see https://musescore.org/en/handbook/3/formatting

That's kind of a big question, so I can answer generally, and then if you need more direct advice, attach your score and explain more specifically what you want to see different.

Here are the general things to know:

• to control the size of the page and margins, use Format / Page Settings
• same dialog to control the size of the music itself (the scaling / staff space setting)
• to add a system break, select a measure and press Enter
• to add a page break break, use Ctrl+Enter
• the Handbook section on "Formatting" is where you can learn more about all of this