Suppose that m is an integer with m > 1 and gcd(a(a-1), m) = 1. Prove that:

1 + a + a^2 + ... + a^(phi(m) - 1) ≡ 0 mod m

I'm really not sure where to start with this. I assume I will have to make use of Euler's Theorem at some point, but I don't even know where to begin.