TLDR; You should start using moustache wax either when the hair begins to overgrow your top lip, or when your moustache is about a centimetre in length and long enough to style.
Moustache wax is used primarily for its ability to hold facial hair in place. This is done by heating up the wax, applying it evenly throughout the moustache, teasing the hair into position and allowing the wax to cool down and strengthen.
I'm always surprised at the number of people who ask me how to grow a moustache. To me, it seemed obvious. However, in retrospect, there are a few things I wish someone had told me.