A is a sharp- it means that the note it's placed next to will be played a half-step up (on a piano, that usually means you go to the closest black key up)
B is a flat- same thing as a sharp, but it's a half step DOWN.
C is a natural- basically if the key you're playing in already has sharps or flats (you can tell by seeing if they're next to the clef at the start of the song) then the natural next to any note tells you to play that one normally, not as a sharp or flat (so on a piano, a white key instead of a black key)
D- is a ledger line- a little line to show where that note goes, even though it's outside of the staff. the one right there would be a C (and the ledger line above it would be an E)
hope you do well on your homework :)