What is the sharp sign?

The sharp sign is written using the following symbol:

It would sit on a music stave (staff USA) as follows:

It indicates that you should play the note one step up (including the black keys on a piano) from the musical note indicated by either the semibreve (whole note USA), minim (half note USA), crochet (quarter note USA) or all the quavers (eighth note USA).

You can recall this by remembering:

The sharp sign (which is very similar to a hashtag sign) looks like a ladder that you can climb up i.e. go up one.

As an example, C sharp or E sharp on a piano would be:

Notice how an E sharp is the F key.

If a sharp, flat or natural appears in one bar of music it lasts the whole bar unless told otherwise.