When writing out the scale you need to know when to use sharps or flats. As you noticed in the C major scale there are no sharps or flats, the scale uses white keys only. When you use the major scale formula beginning on another note such as D, black keys come into play.

(Remember that a sharp is an 1/2 step upward move and a flat is an 1/2 step downward move. Also no keys signatures have both sharps and flats, it's one or the other.)

Pull out your keyboard and play the D major scale. Starting on D go up 1 whole step to the 2nd (E) then another whole step to the 3rd (F). You're sitting on a black key. Now ask yourself, is this black note above or below the F key? It's above, that would make it a sharp. Let's continue onward one half step then three whole steps. Again you find yourself on a black key. It's some kind of C. Is the black key above or below C? It's above, we have another sharp.

The key of D has two sharps. Note that the key signature in the top left of the staff shows what notes are sharp. That's where you look to find what key your playing in.

Starting on F travel up the scale till you get a 4th above the F. You should be on a black key. It is some type of B. Is the black key below or above B? Below, it appears we have a flat here.

As you continue up the scale there will be no more black keys. The key of F has one flat, B flat.

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