There are several symbols that change the sound of the note called SHARPS and FLATS.
Take a look at this picture:
"Natural" Notes
The piano is made up of white key notes and black key notes. All the lines and spaces you learned for treble and bass clef are the white keys on the piano. These are called NATURAL notes because they naturally fall on the lines and spaces on the staff. The black notes between the white notes are sharps and flats.
Whole and Half Steps
The keyboard, or staff, is made up of what we call STEPS. A WHOLE STEP is when you move from one note to another with a note in between it. Look from C to D on the keyboard. See that black key in between it? That means from C to D is a whole step. A HALF STEP is where you move from one note to another without a note in between. Look at C again. The very next black key is a half step. There are two places on the keyboard where a half step happens without a black key - E to F and B to C. There are no black notes between them, so they are considered half steps.
SHARPS
Take a look at the note on the staff below:

The symbol beside the note C is called a SHARP. A sharp is a symbol that tells you to RAISE A NOTE BY HALF A STEP. So if you move from that C on the keyboard by half a step, you would land on the black key just above it (or to the right). That note name would be a C#, or C sharp.
Look at this note:

This is a G sharp (G#), a step above the note G. Important: A sharp symbol ALWAYS GOES ON THE LEFT OF THE NOTE! This is because the symbol is telling you to sharp the note that follows it.
FLATS
If you can raise notes by half a step, then you can also lower them. Take a look at this note:

The symbol beside the B is called a FLAT, meaning the B should be lowerd by ½ a step - Bb, or the black key TO THE LEFT (or going down the keyboard or staff).
Remember, the flat symbol always goes on the left hand side.
REMEMBER
- Going up the keyboard (to the right) or staff, you get sharps.
- Going down the keyboard (to the left), or staff, you get flats.
Sharps and flats ARE NOT affected by time signature, speed, or note value. It simply has to do with the pitch and the note name.