Question: I recently began learning sight singing from Robert Ottman's book.

I've gotten to the point where you should sing simple melodies, with the biggest interval between 2 notes being a major second. The book says that if you have the major scale in your head it should be easy to sight sing these melodies, but I can't. 

Are there any exercises I can do to stick the major scale in my head that would make me progress in this situation?

Answer: 
I suggest you sit at a piano, and slowly play a major scale, but just before you *play* each the note you *sing* the pitch you think you're about to hear.

If you're feeling more ambitious you can do the scale notes out of sequence - but again try to sing the note before you play it. If you find certain intervals hard you can concentrate on those, repeating until you start to feel more confident.

Moving on from single notes, you should also try singing a run of notes from the scale, say the first or last four or five notes, and then check with the piano to see whether you sang the right notes or not.
I'd head straight for the eponymous song 'Do a Deer', from the Sound of Music. Another would be 'Scales and Arpeggios' from the Aristocats.