How should i start off this proof?

If (2^n -1) is prime, prove that n is prime.

So far I've only got 2 things written down:

1. If 2^n -1 is prime then using divisor notation. 1|(2^n -1) and (2^n -1)|(2^n -1). But I dont think this helps.

2. To prove this we can prove gcd((2^n -1),n) = 1 (i.e relatively prime)

Other than that not much idea at all.

Any suggestions would be greatly appreciated.