Sharps and flats are also called accidentals. The symbol for a flat is like a lower case b, while the symbol for a sharp is like a pound sign (#). To flat a note means to lower it by one half step; to sharp a note means to raise it one half step. Accidentals are placed in front of the note they alter. The effect of accidentals last for the entire measure if it is used at the beginning of a measure. Its effect is cancelled by a bar line.
A natural sign is used to cancel out any key that is sharped or flatted. As an example, if a note is in C sharp a natural sign will bring the note back to its natural tone which is C. In the same way, if a note is in F flat, a natural sign will bring back that note to its natural tone which is F.