In order to find the score you're looking for, you need to type its title and/or composer in the search bar at the top of any page on the website and the Browse page in the app. You can also use DuckDuckGo to search musescore.com by adding the bang "!museScore" to your search term.
There is also a variety of sorting options and search filters. On the website, they are available in the sidebar on the left; on mobile devices, they are under the 'Sort' and 'Filter' dropdown menus (located on the right of the search bar).
Sort:
In the mobile app, you can sort the search results based on the number of comments, view count, or rating.
On the website, you can sort by the same options plus upload date or relevance.
Filter: You can filter the search results by
different kinds of instruments which should be present in the score, to exclude results without those;
the number of instruments in the score you're looking for;
ensemble type;
genre;
Keywords: drums, guitar, violin, cello, trumpet.