Accidentals are used to indicate to a player or singer that a note should be raised or lowered by one degree (by a semitone).
They are placed just before a note, and affect that note for the rest of the bar.
At the simple level, there are three accidentals:
Sharps: (). Tells the player/singer to raise the note one semitone higher than it would normally be
Flats: (). Tells the player/singer to lower the note one semitone lower than it would normally be
Naturals: (). This is the slightly odd one, as it just tells the player/singer to leave the note as it should be! Usually used to cancel a previous sharp or flat.
Accidentals can be a bit confusing, not least because some notes have the same name (an A, for instance, is the same as B) but don’t worry – you’ll get used to it soon enough!