How do you write a G major scale?

The "G" in a G major scale is the starting pitch (also: root, tonic). "Major" tells you the steps you take when playing the scale up to the next G. The distance jumped between notes give a scale its character. A major scale sounds like the do-re-mi scale. Other scales, like minor scales, can sound sad.

The "major" refers to the root chord you play. If it's major (G,B,D), the distance between the G and the B (major third) sounds bright. If it's minor (G, Bb, D), the third is flattened, so it tends to sound sadder.

A major scale is made up of a chunk of 3 whole step notes (G, A, B) followed by another chunk of 4 whole step notes (C, D, E, F#). These two chunks are a half step apart. (B-C).

Whole steps are steps with a key wedged between them. For example, G->A is a whole step because there is one black key between them. A half step has no key between notes. B->C is an example. So is F#->G.