What does a flat do? A flat lowers the note a half step. That would cause the note to go from a white key down to a black key or if there is no black key inbetween then directly to the next white key.
We talked before about how important identifying key signatures is to being able to successful complete theory assignments. How do we name the flat key signatures? If there is only one flat then it is the key signature of F. This one is just straight forward memorization. After that you go to the next to the last flat. The order of the flats is:
Bb, Eb, Ab, Db, Gb, Cb, Fb
If your music had three flats, those flats would be: Bb, Eb, Ab and you would be in the key of Eb.
Here is a chart of all the flat key signatures:
Understanding and memorization of the key signatures can cut down a huge number of mistakes and misunderstandings. Take the time to really understand how sharps and flats work and use that information to your advantage.