Hi, I have this question and i'm really struggling.

Show that for any prime p either 2^{p} - 1 is prime or q = 2^{p} - 1 behaves like a prime with respect to the base 2 ie 2^{q-1}\equiv{1} (mod q)

I know this is to do with Fermat's little theorem. But i dont know where to start. Please can someone give me a clue?