Sharp and flat notes are opposites, so the difference between them is very easy to understand: one goes up, the other down. When a note’s pitch is sharpened, it is raised by a semitone (or a half-step). Similarly, when a note’s pitch is flattened, it is lowered by a semitone.
The easiest thing to understand different pitches is to look at a standard piano keyboard. Each key represents a semitone, with the lowest notes on the left of the piano and the highest on the right. So, when a note is sharpened, you move one key up to the right (black or white depending on which is closest) and when a note is flattened you move one key down to the left. Simple as that!
When reading music on the page, sharp or flat notes are shown by symbols that are known as accidentals. These tell you to change the pitch of the original note. A sharp symbol looks like this: ♯ (similar to, but not the same as, the ‘hashtag’ symbol # on social media). A flat symbol looks like this:♭(similar to a lowercase b).
Occasionally, notes can also be double-sharp or double-flat. The premise of these is the same but moves the pitch by two semitones (or a tone). So, if you have a G double sharp, raising this by a tone would make it an A. If you have a G double flat, this would become an F. On a piano keyboard, you would move two keys to the right (sharp) or left (flat).