Key signature is arguably the most important aspect of music, especially when you're first seeing a piece. The key signature notates which/how many sharps or flats are played in a piece. Play a note out of key and it will stick out like a sore thumb. Sometimes the composer uses this (accidentals) to give attention to a note. Breaking the pattern of the key changes the mood. To play sharp, bring the note up a half a step, for example, going from F to F#. To play flat, bring the note down a half a step, from B to Bb. The sharps and flats will always be in a specific order. The good thing is that the order of flats is the order of sharps backwards, so you only need to remember one order very well.
Order of flats: B,E,A,D,G,C,F
Order of sharps: F,C,G,D,A,E,B
Minor keys officially have the same number of sharps and flats, but they are usually variations on the relative major key. There are different relationships between the different keys, this is another important thing to keep in mind. Next time... Common keys
Here is a a convenient chart of the major and minor keys...
Picture:
Behind the music, http://method-behind-the-music.com/theory/scalesandkeys, Ben Hollis,
Kate Bankson, and Chris Gillotti
