F major has one flat (Bb) which falls on the 4th note of the scale. The Bb causes a semitone to be formed between the 3rd and 4th notes of the scale as is the normal pattern in major scales. The semitone from E-F or the 7th and 8th degrees is already there.
 
Important:
In the examples pictured there is a 'key signature' just after the clef. Bb is indicated on the middle line, (2nd line on bass stave) this means that all Bb's's in this music are to be flattened (lowered a semi-tone) You can also be asked in exams to write out the scale without key signature which means placing a flat (or sharp) just before the note.


Listen to F major (roll mouse over image)