Regular Trombones are non-transposing instruments and use bass clef. For this reason, besides trombone sheet music, you can grab any old bassoon part or bass clef baritone part from your school's music collection by asking the band director or the band librarian. In this case, you can find a common melody like Greensleeves in a solo version by doing a web search for "greensleeves sheet music" and browsing until you find a solo or a part with the melody for any of the instruments I already mentioned. You don't usually have to purchase common songs, so keep browsing until you find a free one. For example, you would find something like
this beginner's trombone solo with piano accompaniment. You may need to download free sheet music reader plugins like Scorch to be able to see the image.