Notes that are lowered by half a step (one semitone) are considered "flat".
Indicate a flat note by placing a _ (shift-minus) before the
note name.
play "E _B G B"
Once a note is indicated as flat, that note will continue to be played flat until the end of the measure or line.
Indicate a natural (unflat) note by using =
before the note name:
play "_B =B C' B"
Occassionally a double-flat is needed, and it can be written as __.
Read about key signatures if you are writing music with many flats.