When a note must be raised in pitch by a semitone ( one fret ) the sharp sign is placed directly in front of the note on the staff. In the example below, the second note in each bar has a # ( Sharp ) sign placed in front of it. The note has been changed from a C note to a C# note, and is played one fret higher on the fretboard. The important thing to note ( excuse the pun :) is that the bar line has cancelled the effect of the sharp. That is why the first note in the second bar is a C natural ( a natural just means the note has reverted back to its natural state, not sharp or flat).

In the example below, the first note in the bar has been made sharp. Notice that the effect of the sharp has also changed the second note to a C# as well.The sharp rules until it is cancelled by the bar line.

In this example the second note has a Natural sign in front of it, a Natural also cancels the effect of the accidental (sharp or flat).

This example shows how the same note in octaves above or below the note that is sharped are not affected.

This example illustrates the confusion that can arise by the use of Double Sharps. I have chosen to use the rule that a Natural completely cancels out a Double sharp just as a bar line would. Notice that I have restated the sharp after the natural to make the last note a C#. According to some theory books a double natural is needed to completely cancel a Double Sharp.

This last example shows how sharps in a Key Signature affect all notes regardless of whether they are in the same octave or not.

In case your still pondering why or where you would need to make a note a Double sharp, consider the B7#9 chord.. Which is made from the 1st, 3rd , 5th ,b7th and #9th notes from the B major scale. Using this chord formula the notes from the B Major scale would be; B (1), D# (3rd), F# (5th), A (b7th), and C Double Sharp (#9th) . The ninth (or second) is already a sharp, so in order to raise it another semitone it must be Double Sharped.