Let G = <a> be a cyclic group of order n, and d a divisor of n, show that the number of elements in G of order d is phi(d), where phi is the Euler-phi function.

I'm so confused, I don't really know where to start. I think I can assume that there is a unique subgroup H where |G| = d, because d is a divisor of n.

But I'm kind of absolutely stuck after that.