Before we learn about a sharp let's learn about half-steps.

When you are on the keyboard and play a key, and then you go to the very next note up the keyboard, whether it is white or black, you are said to be moving up a half-step.
If you go to the very next key down, white or black, you are said to be going down a half-step.
Look at the example below and see how B to C is a half-step because the very next note up, white or black from B, is C.

If we start on C and move a half-step up it is actually
the black note immediately right of C.

So what should we call this note that is 1/2 step up from C?

We call it C sharp...read below and it will be explained!
 

Here is a picture of a sharp sign.

When you see this sign it means that the note immediately right of this sign is to be played one half-step up.
The easiest way to remember this is to think that when you sit on a sharp tack,
 you go straight up - and so do "sharps".

A sharp sign is always immediately left of the note.

Here are the name of the sharp keys.

Below is "When The Saints" using a sharp sign.