How to load existing music

If by "existing sheet music" you mean, already in MuseScore format, you just go to File / Open from within MuseScore, or double-click the file in your file browser. If you mean, a MusicXML file (a standard format that most notation programs can produce), File / Open will import it. If you mean PDF, or something printed on paper, then it's basically like asking for the requirements for loading an existing book into a word processor: basically, it means typing it in yourself. With plain text, you can often scan it and use special "Optical Character Recognition" software and maybe get something close to the original. With music this there is "Optical Music Recognition", but it's nowhere near as likely to produce usable results.

Depends on the source, see also: https://musescore.org/en/handbook/3/opensaveexportprint#open-file and: https://musescore.com/import

Are you looking to scan some shet music and have it converted in to a MuseScore file (a bit like putting a pizza into a scanner and getting the recipe printed out) or converting from another music program's files to MuseScore's (possibly a bit more do-able).?