I'm not familiar with proper electrical code, but I think the white wire is usually a neutral/negative, or "return wire". It'll need to be spliced with the other neutral wires coming from your power source. Your black wire is your active/positive, or "supply wire." Of course, this'll need to be spliced into the lead wire coming from the power source. And the green wire is your ground.

If you're going to be splicing directly into a live source, like an attic light, outlet, etc., you should definitely test all of the lead wires first with a voltage reader. Because if someone wired it backwards when your home was being constructed, you wouldn't find out until you wired your fan backwards, powered it up, noticed it wasn't working, and went to touch it. And that's a rough way to have to find out.

If you're not comfortable with matching wires, splicing wires, etc., then you should definitely get someone with experience to wire it for you. If done improperly you could possibly start a fire, or worse, electrocute yourself. I used to install digital audio/video systems in public buildings (conference rooms, auditoriums, schools, churches, etc.) and many times while they were under construction or being renovated. There have been several times that I accidentally grabbed a live wire while at the same time grounding myself to metal rafters. It hurts, really bad. So just be careful if you decide to do it yourself.