Case Fan Installation

So I bought an extra fan for the case (120).

Which way should it be facing(spin left or right)?Should it suck air in or out?

And also would mounting it on the front of the comp(hard disk) make better air flow compared to mounting it on the side (facing GPU)?

Tnx for reply(s)!

What if I put it on the side?

EDIT: Doing this for a friend at job,whos even a bigger noob than I am and I don`t have much time to read stuff on the net..