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: 
When it comes to basic building blocks like the major scale, it's often best to learn it by playing it on a piano or other instrument until you can mimic the sound yourself.

As for repertoire, the opening of ["Joy to the World"](https://www.youtube.com/watch?v=O5hj518Iugk) is a great *descending* major scale. Try singing this before the beginning of every Ottman exercise to get yourself into the key and to start conceptualizing the major-scale space.
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.